v1.3: New features and many bug fixes.

New features include:
- file.flush, os.stat
- unicode enabled by default on unix and stmhal ports
- print function accepts file= argument
- I2C object allows for 16-bit mem addr
- emergency exception buffer for improved exception traceback
- USB_VCP object
- 0:/ and 1:/ changed to /flash and /sd
- improved Pin object
- can set pin alternate function from Python
- builtin reversed, can len() and subscr range objects
- added moductypes
- added modzlibd for DEFLATE decompression

Code improvements include:
- more efficient line-number to bytecode mapping
- new STM HAL driver v1.3.0

Bug fixes include:
- mpz (bignum) fixes
- accel startup improved
- important bug fix in GC