Implement default function arguments (for Python functions).

TODO: Decide if we really need separate bytecode for creating functions
with default arguments - we would need same for closures, then there're
keywords arguments too. Having all combinations is a small exponential
explosion, likely we need just 2 cases - simplest (no defaults, no kw),
and full - defaults & kw.
Merge branch 'fun-defargs' of github.com:pfalcon/micropython into pfalcon-fun-defargs
py: mp_execute_byte_code has 2 arg arrays, for more efficient default params.