For consistency this helper function is renamed to match the other
exception helpers, and moved to their location in runtime.c.

Header files that are considered internal to the py core and should not
normally be included directly are:
    py/nlr.h - internal nlr configuration and declarations
    py/bc0.h - contains bytecode macro definitions
    py/runtime0.h - contains basic runtime enums

Instead, the top-level header files to include are one of:
    py/obj.h - includes runtime0.h and defines everything to use the
        mp_obj_t type
    py/runtime.h - includes mpstate.h and hence nlr.h, obj.h, runtime0.h,
        and defines everything to use the general runtime support functions

Additional, specific headers (eg py/objlist.h) can be included if needed.

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

Useful for embedded targets with fixed stack layout.

I.e. in this mode, C stack will never be used to call a Python function,
but if there's no free heap for a call, it will be reported as
RuntimeError (as expected), not MemoryError.

So corresponding exception can be thrown even under tight memory conditions.

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.

Part of code cleanup, working towards resolving issue #50.

Such mechanism is important to get stable Python functioning, because Python
function calling is handled with C stack. The idea is to sprinkle
STACK_CHECK() calls in places where there can be C recursion.

TODO: Add more STACK_CHECK()'s.

This removes need for some casts (at least, more than it adds need
for new casts!).

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/.

Make include dependencies neater, and adheres to the coding convention
that headers should not include headers.

This does not affect code size or performance when debugging turned off.

To address issue #420.

Adds readline_init() to clear readline history on soft reset.  Addresses
issue #387.