Make generator_stop universal

The idea is to support [PEP 479](https://www.python.org/dev/peps/pep-0479/) on all versions (not just those which support `from __future__ import generator_stop`, which is the current behavior).

Start by reading PEP 479 and tracing Coconut’s existing handling of `from __future__ import generator_stop`. Determine how generator_stop should behave on Python versions without that future feature, then verify the resulting generator behavior across the supported versions. Done means PEP 479 behavior is universal rather than conditional on future-import support.

Written by the indexing model from the issue text.