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.