stmhal: in EXTI interrupt handler wrap uPy calls in gc_lock and nlr_buf.
py: Improve compiler syntax errors; catch more errors.
py: Raise exception for unimplemented byte codes.
py: implement UNPACK_EX byte code (for: a, *b, c = d)
py: Improve inline assembler; add a few more opcodes.
Merge branch 'master' of github.com:micropython/micropython