The esp register is always a fixed distance below ebp, and using esp to
reference locals on the stack frees up the ebp register for general purpose
use (which is important for an architecture with only 8 user registers).

Instead of storing the function pointer directly in the assembly code.
This makes the generated code more independent of the runtime (so easier to
relocate the code), and reduces the generated code size.

The rsp register is always a fixed distance below rbp, and using rsp to
reference locals on the stack frees up the rbp register for general purpose
use.

For s132 and s140, GAP_ADV_MAX_SIZE was currently set to
BLE_GATT_ATT_MTU_DEFAULT, which is 23. The correct value
should have been 31, but there are no define for this in
the s132/s140 header files as for s110.

Updating define in ble_drv.c to the correct value of 31.

The macros are MICROPY_HEAP_START and MICROPY_HEAP_END, and if not defined
by a board then the default values will be used (maximum heap from SRAM as
defined by linker symbols).

As part of this commit the SDRAM initialisation is moved to much earlier in
main() to potentially make it available to other peripherals and avoid
re-initialisation on soft-reboot.  On boards with SDRAM enabled the heap
has been set to use that.

Add some more POSIX compatibility by adding a d_type field to the
dirent structure and defining corresponding macros so listdir_next
in the unix' port modos.c can use it, end result being uos.ilistdir
now reports the file type.

This value is unused. It was an artifact of early draft design, but
bitfields were optimized to use scalar one-word encoding, to allow
compact encoding of typical multiple bitfields in MCU control
registers.

E.g., register() can be called again for the same object, while modify()
will raise exception if object was not register()ed before.

This test doesn't check the actual I/O behavior, just "static" invariants
like behavior on duplicate calls or calls when I/O object is not registered
with poller.

Previously, the function silently succeeded. The new behavior is consistent
with both baremetal uselect implementation and CPython 3.

As the case for e.g. Android's Bionic Libc.

This follows how it's already done in MP_OBJ_IS_OBJ: the objects are
considered 64-bit unsigned ints for the purpose of bitwise manipulation.

A non-zero level has been supported for some time now.

With this commit there is now only one entry point into the whole
documentation, which describes the general MicroPython language, and then
from there there are links to information about specific platforms/ports.

This commit doesn't change content (almost, it does fix a few internal
links), it just reorganises things.

This commit adds first class support for yield and yield-from in the native
emitter, including send and throw support, and yields enclosed in exception
handlers (which requires pulling down the NLR stack before yielding, then
rebuilding it when resuming).

This has been fully tested and is working on unix x86 and x86-64, and
stm32.  Also basic tests have been done with the esp8266 port.  Performance
of existing native code is unchanged.

The nlr_buf_t doesn't need to be part of the Python value stack (as it was
before this commit), it's simpler to have it separated as auxiliary state
that lives on the C stack.  This will help adding yield support because in
that case the nlr_buf_t and Python value stack live in separate memory
areas (C stack and heap respectively).

Instead of hard-coding it to 4 bytes.  This allows for there to be other
data stored at the very start of the emitted native code.

In particular this allows native functions on Thumb2 to index more than 32
constants in the constant table.

Instead of at end of state, n_state - 1.  It was originally (way back in
v1.0) put at the end of the state because the VM didn't have a pointer to
the start.  But now that the VM takes a mp_code_state_t pointer it does
have a pointer to the start of the state so can put the exception object
there.

This commit saves about 30 bytes of code on all architectures, and, more
importantly, reduces C-stack usage by a couple of words (8 bytes on Thumb2
and 16 bytes on x86-64) for every (non-generator) call of a bytecode
function because fun_bc_call no longer needs to remember the n_state
variable.

This makes these special methods have the same calling behaviour as other
methods in a class instance (mp_convert_member_lookup() is already called
by mp_obj_class_lookup()).

This behaviour is tested in basics/class_store.py and follows CPython.

And remove related comment about needing such protection when calling send.

Reasoning for removal is as follows:
- mp_resume is only called by the VM in YIELD_FROM opcode
- if send_value != MP_OBJ_NULL then throw_value == MP_OBJ_NULL
- so if __next__ or send are called then throw_value == MP_OBJ_NULL
- if __next__ or send raise an exception without nlr protection then the
  exception will be handled by the global exception handler of the VM
- this handler already has code to handle exceptions raised in YIELD_FROM,
  including correct handling of StopIteration
- this handler doesn't handle the case of injection of GeneratorExit, but
  this won't be needed because throw_value == MP_OBJ_NULL

Note that it's already possible for mp_resume() to raise an exception
(including StopIteration) from the unprotected call to type->iternext(), so
that's why the VM already has code to handle the case of exceptions coming
out of mp_resume().

This commit reduces code size by a bit, and significantly reduces C stack
usage when using yield-from, from 88 bytes down to 40 for Thumb2, and 152
down to 72 bytes for x86-64 (better than half).  (Note that gcc doesn't
seem to tail-call optimise the call from mp_resume() to mp_obj_gen_resume()
so this saving in C stack usage helps all uses of yield-from.)

mp_make_raise_obj must be used to convert a possible exception type to an
instance object, otherwise the VM may raise a non-exception object.

An existing test is adjusted to test this case, with the original test
already moved to generator_throw.py.

This matches how bytecode does it, and matches the signature of
mp_emit_glue_assign_native.  Since the native emitter doesn't support
nan-boxing uintptr_t and mp_uint_t are anyway the same bit-width.

The native code no longer holds live GC pointers so doesn't need to be
scanned.

After the previous commit this macro is no longer needed by the native
emitter because live heap pointers are no longer stored in generated native
machine code.

This commit changes native code to handle constant objects like bytecode:
instead of storing the pointers inside the native code they are now stored
in a separate constant table (such pointers include objects like bignum,
bytes, and raw code for nested functions).  This removes the need for the
GC to scan native code for root pointers, and takes a step towards making
native code independent of the runtime (eg so it can be compiled offline by
mpy-cross).

Note that the changes to the struct scope_t did not increase its size: on a
32-bit architecture it is still 48 bytes, and on a 64-bit architecture it
decreased from 80 to 72 bytes.