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.

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.

It seems most sensible to use size_t for measuring "number of bytes" in
malloc and vstr functions (since that's what size_t is for).  We don't
use mp_uint_t because malloc and vstr are not Micro Python specific.

It defines types used by all other headers.

Fixes #691.

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/.

This is a small optimisation to zero out only the additional bytes that
the caller did not ask for.

Also add some more debugging output to gc_dump_alloc_table().

Now that newly allocated heap is always zero'd, maybe we just make this
a policy for the uPy API to keep it simple (ie any new implementation of
memory allocation must zero all allocations).  This follows the D
language philosophy.

Before this patch, a previously used memory block which had pointers in
it may still retain those pointers if the new user of that block does
not actually use the entire block.  Eg, if I want 5 blocks worth of
heap, I actually get 8 (round up to nearest 4).  Then I never use the
last 3, so they keep their old values, which may be pointers pointing to
the heap, hence preventing GC.

In rare (or maybe not that rare) cases, this leads to long, unintentional
"linked lists" within the GC'd heap, filling it up completely.  It's
pretty rare, because you have to reuse exactly that memory which is part
of this "linked list", and reuse it in just the right way.

This should fix issue #522, and might have something to do with
issue #510.

Previously, a failed malloc/realloc would throw an exception, which was
not caught.  I think it's better to keep the parser free from NLR
(exception throwing), hence this patch.

A malloc/realloc fail now throws MemoryError.

Remove unnecessary includes. Add includes that improve portability.

In particular, unix outputs to stderr, to allow to run testsuite against
micropython built with debug output (by redirecting stderr to /dev/null).

If there's malloc and memset, then there's no need for calloc, especially if
we need to implement it ourselves.

To alloc complete memory alloc flow tracing.

mpconfig.h will automatically pull mpconfigport.h.

This is just max value of current_bytes_allocated seen.

Unlike total_bytes_allocated, this tracks m_free()'s too.