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.