Previously, trace hooks running on a nested function
could incorrectly reset the state of a closure cell.

This avoids that by slightly changing the semantics
of the locals namespace in functions, generators,
and coroutines, such that closure references are
represented in the locals namespace by their
actual cell objects while a trace hook implemented
in Python is running.

Depends on PEP 558 and bpo-17960 to cover the
underlying change to frame.f_locals semantics.