py: Fix some bugs in mpz; add mpz_from_ll and mpz_set_from_ll.

A couple of bugs in mpn_shl, and overflow bug in mpz_set_from_int.
py: Implement integer overflow checking for * and << ops.

If operation will overflow, a multi-precision integer is created.
Add more tests for multi-precision integers.
Merge pull request #339 from dhylands/stmhal

Initial checkin with STM HAL
Fix realloc
py: Add expm1 to math module.
stmhal: Get SysTick and HSE working.
stmhal: Set SysTick priority to highest level.
stm: Add some floating-point math functions.

These have been taken from the musl library.  It's a bit of a hack for
now, just to get math functions working.
Merge pull request #340 from iabdalkader/realloc

Fix realloc
py: Cosmetic changes.
windows: Change MICROPY_FLOAT config to new scheme.
REPl working on UART6 with STMHAL