modsocket: 64-bit cleanness.
Rename configuration variables controling Python features.

Now of the form MICROPY_PY_*.  See issue #35.
Add SystemExit exception and use it in unix/ and stmhal/ ports.

Addresses issue #598.
objslice: Support arbitrary objects start, stop, and step.

Older int-only encoding is not expressive enough to support arbitrary slice
assignment operations.
py: Handle case of slice start > stop in common sequence function.
objlist: Implement growing slice assignment.

This means that complete slice operations are supported for lists (but not
for bytearray's and array.array's).
sequence: Throw exception for not implemented slice steps.
py: Refactor slice helpers, preparing to support arbitrary slicing.
objlist: Implement support for arbitrary (3-arg) slices.