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.

In unix port, mem_info(1) now prints pretty GC alloc table.

See discussion in issue #50.

Step 1 fixes #732

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/.