Having balanced braces { and } makes it easier to navigate the function.

When pystack is enabled mp_obj_fun_bc_prepare_codestate() will always
return a valid pointer, and if there is no more pystack available then it
will raise an exception (a RuntimeError).  So having pystack enabled with
stackless enabled automatically gives strict stackless mode.  There is
therefore no need to have code for strict stackless mode when pystack is
enabled, and this patch optimises the VM for such a case.

Neither the ip nor sp variables are used again after the execution of the
RAISE_VARARGS opcode, so they don't need to be updated.

There's no need to have MP_OBJ_NULL a special case, the code can re-use
the MP_OBJ_STOP_ITERATION value to signal the special case and the VM can
detect this with only one check (for MP_OBJ_STOP_ITERATION).

This patch concerns the handling of an NLR-raised StopIteration, raised
during a call to mp_resume() which is handling the yield from opcode.

Previously, commit 6738c1dd introduced code
to handle this case, along with a test.  It seems that it was lucky that
the test worked because the code did not correctly handle the stack pointer
(sp).

Furthermore, commit 79d996a5 improved the
way mp_resume() propagated certain exceptions: it changed raising an NLR
value to returning MP_VM_RETURN_EXCEPTION.  This change meant that the
test introduced in gen_yield_from_ducktype.py was no longer hitting the
code introduced in 6738c1dd.

The patch here does two things:

1. Fixes the handling of sp in the VM for the case that yield from is
   interrupted by a StopIteration raised via NLR.

2. Introduces a new test to check this handling of sp and re-covers the
   code in the VM.

This constant exception instance was once used by m_malloc_fail() to raise
a MemoryError without allocating memory, but it was made obsolete long ago
by 3556e457.  The functionality is now
replaced by the use of mp_emergency_exception_obj which lives in the global
uPy state, and which can handle any exception type, not just MemoryError.

This patch simplifies how sentinel values are stored on the stack when
doing an unwind return or jump.  Instead of storing two values on the stack
for an unwind jump it now stores only one: a negative small integer means
unwind-return and a non-negative small integer means unwind-jump with the
value being the number of exceptions to unwind.  The savings in code size
are:

   bare-arm:   -56
minimal x86:   -68
   unix x64:   -80
unix nanbox:    -4
      stm32:   -56
     cc3200:   -64
    esp8266:   -76
      esp32:  -156

With MICROPY_ENABLE_PYSTACK enabled the following language constructs no
longer allocate on the heap: f(*arg), f(**kwarg).

For consistency this helper function is renamed to match the other
exception helpers, and moved to their location in runtime.c.

Header files that are considered internal to the py core and should not
normally be included directly are:
    py/nlr.h - internal nlr configuration and declarations
    py/bc0.h - contains bytecode macro definitions
    py/runtime0.h - contains basic runtime enums

Instead, the top-level header files to include are one of:
    py/obj.h - includes runtime0.h and defines everything to use the
        mp_obj_t type
    py/runtime.h - includes mpstate.h and hence nlr.h, obj.h, runtime0.h,
        and defines everything to use the general runtime support functions

Additional, specific headers (eg py/objlist.h) can be included if needed.

For consistency with all the other exception messages.

There were several different spellings of MicroPython present in comments,
when there should be only one.

It's not used anywhere else in the VM loop, and clashes with (is shadowed
by) the n_state variable that's redeclared towards the end of the
mp_execute_bytecode function.  Code size is unchanged.

Taking the address of a local variable leads to increased stack usage, so
the mp_decode_uint_skip() function is added to reduce the need for taking
addresses.  The changes in this patch reduce stack usage of a Python call
by 8 bytes on ARM Thumb, by 16 bytes on non-windowing Xtensa archs, and by
16 bytes on x86-64.  Code size is also slightly reduced on most archs by
around 32 bytes.

This patch fixes a regression introduced by
088740ec

This patch fixes a regression introduced by
71a3d6ec

Previous to this patch the n_state variable was referring to that computed
at the very start of the mp_execute_bytecode function.  This patch fixes it
so that n_state is recomputed when the code_state changes.

This patch allows the following code to run without allocating on the heap:

    super().foo(...)

Before this patch such a call would allocate a super object on the heap and
then load the foo method and call it right away.  The super object is only
needed to perform the lookup of the method and not needed after that.  This
patch makes an optimisation to allocate the super object on the C stack and
discard it right after use.

Changes in code size due to this patch are:

   bare-arm: +128
    minimal: +232
   unix x64: +416
unix nanbox: +364
     stmhal: +184
    esp8266: +340
     cc3200: +128

Also const_table is now moved to the code_state->fun_bc structure.

To improve maintainability of the code.

It improves readability of code and reduces the chance to make a mistake.

This patch also fixes a bug with nan-boxing builds by rounding up the
calculation of the new NSLOTS variable, giving the correct number of slots
(being 4) even if mp_obj_t is larger than the native machine size.

The scheduler being locked general means we are running a scheduled
function, and switching to another thread violates that, so don't switch in
such a case (even though we technically could).

And if we are running a scheduled function then we want to finish it ASAP,
so we shouldn't switch to another thread.

Furthermore, ports with threading enabled will lock the scheduler during a
hard IRQ, and this patch to the VM will make sure that threads are not
switched during a hard IRQ (which would crash the VM).

This has a noticeable improvement on x86-64 and Thumb2 archs, where stack
usage is reduced by 2 machine words in the VM.

Instead of caching data that is constant (code_info, const_table and
n_state), store just a pointer to the underlying function object from which
this data can be derived.

This helps reduce stack usage for the case when the mp_code_state_t
structure is stored on the stack, as well as heap usage when it's stored
on the heap.

The downside is that the VM becomes a little more complex because it now
needs to derive the data from the underlying function object.  But this
doesn't impact the performance by much (if at all) because most of the
decoding of data is done outside the main opcode loop.  Measurements using
pystone show that little to no performance is lost.

This patch also fixes a nasty bug whereby the bytecode can be reclaimed by
the GC during execution.  With this patch there is always a pointer to the
function object held by the VM during execution, since it's stored in the
mp_code_state_t structure.

The extra 4 slots correspond to the iterator object stored on the stack.

So that the "for x in it: ..." statement can now work without using the
heap (so long as the iterator argument fits in an iter_buf structure).

Allows to iterate over the following without allocating on the heap:
- tuple
- list
- string, bytes
- bytearray, array
- dict (not dict.keys, dict.values, dict.items)
- set, frozenset

Allows to call the following without heap memory:
- all, any, min, max, sum

TODO: still need to allocate stack memory in bytecode for iter_buf.

This improves efficiency of GIL release within the VM, by only doing the
release after a fixed number of jump-opcodes have executed in the current
thread.

When an exception is raised and is to be handled by the VM, it is stored
on the Python value stack so the bytecode can access it.  CPython stores
3 objects on the stack for each exception: exc type, exc instance and
traceback.  uPy followed this approach, but it turns out not to be
necessary.  Instead, it is enough to store just the exception instance on
the Python value stack.  The only place where the 3 values are needed
explicitly is for the __exit__ handler of a with-statement context, but
for these cases the 3 values can be extracted from the single exception
instance.

This patch removes the need to store 3 values on the stack, and instead
just stores the exception instance.

Code size is reduced by about 50-100 bytes, the compiler and VM are
slightly simpler, generate bytecode is smaller (by 2 bytes for each try
block), and the Python value stack is reduced in size for functions that
handle exceptions.

With the previous patch combining 3 emit functions into 1, it now makes
sense to also combine the corresponding VM opcodes, which is what this
patch does.  This eliminates 2 opcodes which simplifies the VM and reduces
code size, in bytes: bare-arm:44, minimal:64, unix(NDEBUG,x86-64):272,
stmhal:92, esp8266:200.  Profiling (with a simple script that creates many
list/dict/set comprehensions) shows no measurable change in performance.

Also at _t to mp_exc_stack pre-declaration in struct typedef.