Replace individual slots in PyFutureFeatures with a single bitmask
with one field per feature. The flags for this bitmask are the same
as the flags used in the co_flags slot of a code object.
XXX This means we waste several bits, because they are used
for co_flags but have no meaning for future statements. Don't
think this is an issue.
Remove the NESTED_SCOPES_DEFAULT define and others. Not sure what
they were for anyway.
Remove all the PyCF_xxx flags, but define PyCF_MASK in terms of the
CO_xxx flags that are relevant for this release.
Change definition of PyCompilerFlags so that cf_flags matches
co_flags.
Jeremy Hylton
fdd12f66bb