Thanks to @robert-hh.

We can actually handle interrupts during a thread switch (because we always
have a valid stack), but only if those interrupts don't access any of the
thread state (because the state may not correspond to the stack pointer).
So to be on the safe side we disable interrupts during the very short
period of the thread state+stack switch.

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).

MHLSB and MHMSB formats are added to the framebuf module, which have 8
adjacent horizontal pixels represented in a single byte.

Since eaef6b53 writes are used instead of
reads.

All arguments to pin.irq are converted from keyword-only to positional, and
can still be specified by keyword so it's a backwards compatible change.

The default value for the "trigger" arg is changed from 0 (no trigger)
to rising+falling edge.

ExtInt, Timer and CAN IRQ callbacks are made to work with the scheduler.
They are still hard IRQs by default, but one can now call
micropython.schedule within the hard IRQ to schedule a soft callback.

Instead of always reporting some object cannot be implicitly be converted
to a 'str', even when it is a 'bytes' object, adjust the logic so that
when trying to convert str to bytes it is shown like that.
This will still report bad implicit conversion from e.g. 'int to bytes'
as 'int to str' but it will not result in the confusing
'can't convert 'str' object to str implicitly' anymore for calls like
b'somestring'.count('a').

* Fix mis-spelling of `ticks_add` in code examples.
* Be consistent about parentheses after function names.
* Be consistent about formatting of function, variable and constant names.
* Be consistent about spaces and punctuation.
* Fix some language errors (missing or wrong words, wrong word order).
* Keep line length under 90 chars.

Signed-off-by: Christopher Arndt <chris@chrisarndt.de>

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 RHS can only be returned if it is the same type as the LHS.

This allows to test the PYBV11 target as well as the network drivers
without adding another rule.  It also removes the need to use -B,
side-stepping the issue of whether or not -B works with qstr auto
generation.

When make is passed "-B" it seems that everything is considered out-of-date
and so $? expands to all prerequisites.  Thus there is no need for a
special check to see if $? is emtpy.

Assertions are used to check expressions that should always be true,
and mp_not_implemented is used for code that can be reached.

This is so the test can run successfully on targets that already have
something mounted.

The 'S' typecode is a uPy extension so it should be grouped with the other
extension (namely 'O' typecode).  Testing 'S' needs uctypes which is an
extmod module and not always available, so this test is made optional and
will only be run on ports that have (u)struct and uctypes.  Otherwise it
will be silently skipped.

Some stack is allocated to format ints, and when the int implementation uses
long-long there should be additional stack allocated compared with the other
cases.  This patch uses the existing "fmt_int_t" type to determine the
amount of stack to allocate.