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).

Fix gc_realloc to expand in place

* Issue #322

unix: Allow to set heap size using "-X heapsize=N" option.

Addresses issue #327.

Checks for non-default args following default args, and errors out.
Addresses issue #328.