Enumerates CDC device over USB, but doesn't transmit/receive yet.

Implement str.count and add tests for it.

Cleanup systick for stmhal

REPL working on UART6 with STMHAL

See pull request #343.

Also modify mp_get_index to accept:
1. Indices that are or evaluate to a boolean.
2. Slice indices.
Add tests for these two cases.

Fix realloc

These have been taken from the musl library.  It's a bit of a hack for
now, just to get math functions working.

Initial checkin with STM HAL

If operation will overflow, a multi-precision integer is created.

A couple of bugs in mpn_shl, and overflow bug in mpz_set_from_int.

This compiles and links, but hasn't been tested on a board
yet and even if it was run, it doesn't currently do anything.

MICROPY_ENABLE_FLOAT is automatically set in mpconfig.h if MICROPY_FLOAT_IMPL
is set to a non-zero value.

Fix makeqstrdata.py to work in Python 2.7

mp_module_obj_t can now be put in ROM.

Configuration of float type is now similar to longint: can now choose
none, float or double as the implementation.

math module has basic math functions.  For STM port, these are not yet
implemented (they are just stub functions).