This comment style is no longer used because the docs are written by hand,
not generated.

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

One can instead lookup __name__ in the modules dict to get the value.

Currently, MicroPython runs GC when it could not allocate a block of memory,
which happens when heap is exhausted. However, that policy can't work well
with "inifinity" heaps, e.g. backed by a virtual memory - there will be a
lot of swap thrashing long before VM will be exhausted. Instead, in such
cases "allocation threshold" policy is used: a GC is run after some number of
allocations have been made. Details vary, for example, number or total amount
of allocations can be used, threshold may be self-adjusting based on GC
outcome, etc.

This change implements a simple variant of such policy for MicroPython. Amount
of allocated memory so far is used for threshold, to make it useful to typical
finite-size, and small, heaps as used with MicroPython ports. And such GC policy
is indeed useful for such types of heaps too, as it allows to better control
fragmentation. For example, if a threshold is set to half size of heap, then
for an application which usually makes big number of small allocations, that
will (try to) keep half of heap memory in a nice defragmented state for an
occasional large allocation.

For an application which doesn't exhibit such behavior, there won't be any
visible effects, except for GC running more frequently, which however may
affect performance. To address this, the GC threshold is configurable, and
by default is off so far. It's configured with gc.threshold(amount_in_bytes)
call (can be queries without an argument).

Without mp_sys_path and mp_sys_argv in the root pointer section of the
state, their memory was being incorrectly collected by GC.

This patch consolidates all global variables in py/ core into one place,
in a global structure.  Root pointers are all located together to make
GC tracing easier and more efficient.

Addresses issue #1022.

This is just a clean-up of the code.  Generated code is exactly the
same.

gc.enable/disable are now the same as CPython: they just control whether
automatic garbage collection is enabled or not.  If disabled, you can
still allocate heap memory, and initiate a manual collection.

Addresses issue #724.

See discussion in issue #50.

Return free/allocated memory on GC heap.

It defines types used by all other headers.

Fixes #691.

Now of the form MICROPY_PY_*.  See issue #35.

Blanket wide to all .c and .h files.  Some files originating from ST are
difficult to deal with (license wise) so it was left out of those.

Also merged modpyb.h, modos.h, modstm.h and modtime.h in stmhal/.

Towards addressing issue #424.

Had a small increase to ROM usage (order 60 bytes).

So far just includes "open" function, which should be supplied by a port.

TODO: Make the module #ifdef'ed.