A self-contained piece of Michael Hudson's patch

A self-contained piece of Michael Hudson's patch
    #449043 supporting __future__ in simulated shells
in support of PEP 264.

Much has changed from the patch version:
+ Repaired bad hex constant for nested_scopes.
+ Defined symbolic CO_xxx names so global search will find these uses.
+ Made the exported list of feature names explicit, instead of abusing
  __all__ for this purpose (and redefined __all__ accordingly).
+ Added gross .compiler_flag verification to test___future__.py, and
  reworked it a little to make use of the newly exported explicit list
  of feature names.