The issue described in the comment added here can be seen by forcing a
gc_collect() at the start of each call to gc_alloc().

In case (user) source code contains utf-8 encoded data and the default
locale is not utf-8.

See #4592.

This means the schedule operates on a first-in, first-executed manner
rather than the current last-in, first executed.

In 1808b2e8 it was replaced by MICROPY_VFS
and related code.

How to use this feature is documented in docs/develop/cmodules.rst.

This system makes it a lot easier to include external libraries as static,
native modules in MicroPython.  Simply pass USER_C_MODULES (like
FROZEN_MPY_DIR) as a make parameter.

During make, makemoduledefs.py parses the current builds c files for
MP_REGISTER_MODULE(module_name, obj_module, enabled_define)

These are used to generate a header with the required entries for
"mp_rom_map_elem_t mp_builtin_module_table[]" in py/objmodule.c

This commit adds support for saving and loading .mpy files that contain
native code (native, viper and inline-asm).  A lot of the ground work was
already done for this in the form of removing pointers from generated
native code.  The changes here are mainly to link in qstr values to the
native code, and change the format of .mpy files to contain native code
blocks (possibly mixed with bytecode).

A top-level summary:

- @micropython.native, @micropython.viper and @micropython.asm_thumb/
  asm_xtensa are now allowed in .py files when compiling to .mpy, and they
  work transparently to the user.

- Entire .py files can be compiled to native via mpy-cross -X emit=native
  and for the most part the generated .mpy files should work the same as
  their bytecode version.

- The .mpy file format is changed to 1) specify in the header if the file
  contains native code and if so the architecture (eg x86, ARMV7M, Xtensa);
  2) for each function block the kind of code is specified (bytecode,
  native, viper, asm).

- When native code is loaded from a .mpy file the native code must be
  modified (in place) to link qstr values in, just like bytecode (see
  py/persistentcode.c:arch_link_qstr() function).

In addition, this now defines a public, native ABI for dynamically loadable
native code generated by other languages, like C.

n_obj no longer includes a count for mp_fun_table to make it a bit simpler.

Simplifies the code and fixes handling of the Ellipsis const in native code
generation (which also needs the constant table so must set this flag).

The new compile-time option is MICROPY_DEBUG_MP_OBJ_SENTINELS, disabled by
default.  This is to allow finer control of whether this debugging feature
is enabled or not (because, for example, this setting must be the same for
mpy-cross and the MicroPython main code when using native code generation).

When encoded in the mpy file, if qstr <= QSTR_LAST_STATIC then store two
bytes: 0, static_qstr_id.  Otherwise encode the qstr as usual (either with
string data or a reference into the qstr window).

Reduces mpy file size by about 5%.

Instead of emitting two bytes in the bytecode for where the linked qstr
should be written to, it is now replaced by the actual qstr data, or a
reference into the qstr window.

Reduces mpy file size by about 10%.

This is an implementation of a sliding qstr window used to reduce the
number of qstrs stored in a .mpy file.  The window size is configured to 32
entries which takes a fixed 64 bytes (16-bits each) on the C stack when
loading/saving a .mpy file.  It allows to remember the most recent 32 qstrs
so they don't need to be stored again in the .mpy file.  The qstr window
uses a simple least-recently-used mechanism to discard the least recently
used qstr when the window overflows (similar to dictionary compression).
This scheme only needs a single pass to save/load the .mpy file.

Reduces mpy file size by about 25% with a window size of 32.

POP_BLOCK and POP_EXCEPT are now the same, and are always followed by a
JUMP.  So this optimisation reduces code size, and RAM usage of bytecode by
two bytes for each try-except handler.

After the previous commit it is no longer needed.

This patch fixes a bug in the VM when breaking within a try-finally.  The
bug has to do with executing a break within the finally block of a
try-finally statement.  For example:

    def f():
        for x in (1,):
            print('a', x)
            try:
                raise Exception
            finally:
                print(1)
                break
            print('b', x)
    f()

Currently in uPy the above code will print:

    a 1
    1
    1
    segmentation fault (core dumped)  micropython

Not only is there a seg fault, but the "1" in the finally block is printed
twice.  This is because when the VM executes a finally block it doesn't
really know if that block was executed due to a fall-through of the try (no
exception raised), or because an exception is active.  In particular, for
nested finallys the VM has no idea which of the nested ones have active
exceptions and which are just fall-throughs.  So when a break (or continue)
is executed it tries to unwind all of the finallys, when in fact only some
may be active.

It's questionable whether break (or return or continue) should be allowed
within a finally block, because they implicitly swallow any active
exception, but nevertheless it's allowed by CPython (although almost never
used in the standard library).  And uPy should at least not crash in such a
case.

The solution here relies on the fact that exception and finally handlers
always appear in the bytecode after the try body.

Note: there was a similar bug with a return in a finally block, but that
was previously fixed in b7352084

Also, to make it possible for ports to provide their own lwipopts.h, the
default include directory of extmod/lwip-include is no longer added and
instead a port should now make sure the correct include directory is
included in the list (can still use extmod/lwip-include).

This optimisation eliminates the need to create a temporary normal dict.
The optimisation is enabled via MICROPY_COMP_CONST_LITERAL which is enabled
by default (although only has an effect if OrderdDict is enabled).

Thanks to @pfalcon for the initial idea and implementation.

All exceptions that unwind through the async-with must be caught and
BaseException is the top-level class, which includes Exception and others.

Fixes issue #4552.

Q_GET_HASH may evaluate its argument more than once.

As mentioned in #4450, `websocket` was experimental with a single intended
user, `webrepl`. Therefore, we'll make this change without a weak
link `websocket` -> `uwebsocket`.

This change makes it so that python3 is required by default to build
MicroPython. Python 2 can be used by specifying make PYTHON=python2.

This comes about due to a recent-ish change to PEP 394 that makes the
python command more optional than before (even with Python 2 installed);
see https://github.com/python/peps/commit/cd59ec03c8ff1e75089d5872520cd0706774b35b#diff-1d22f7bd72cbc900670f058b1107d426

Since the command python is no longer required to be provided by a
distribution we need to use either python2 or python3 as commands.  And
python3 seems the obvious choice.

These macros could in principle be (inline) functions so it makes sense to
have them lower case, to match the other C API functions.

The remaining macros that are upper case are:
- MP_OBJ_TO_PTR, MP_OBJ_FROM_PTR
- MP_OBJ_NEW_SMALL_INT, MP_OBJ_SMALL_INT_VALUE
- MP_OBJ_NEW_QSTR, MP_OBJ_QSTR_VALUE
- MP_OBJ_FUN_MAKE_SIG
- MP_DECLARE_CONST_xxx
- MP_DEFINE_CONST_xxx

These must remain macros because they are used when defining const data (at
least, MP_OBJ_NEW_SMALL_INT is so it makes sense to have
MP_OBJ_SMALL_INT_VALUE also a macro).

For those macros that have been made lower case, compatibility macros are
provided for the old names so that users do not need to change their code
immediately.