py: Rename and reorder parameters in emit_make_function/closure.

In preparation for implementing default keyword arguments.
py: Add LOAD_NULL bytecode and use it to simplify function calls.

Adding this bytecode allows to remove 4 others related to
function/method calls with * and ** support.  Will also help with
bytecodes that make functions/closures with default positional and
keyword args.
Merge branch 'master' of github.com:micropython/micropython