py: Add keyword arg support to enumerate constructor.

Need to have a policy as to how far we go adding keyword support to
built ins.  It's nice to have, and gets better CPython compatibility,
but hurts the micro nature of uPy.

Addresses issue #577.
Merge branch 'master' of https://github.com/micropython/micropython

Conflicts:
	py/argcheck.c
	py/objenumerate.c
	py/runtime.h