Test symbolic constants with 'is (not)'

Saves a few % of total parsing time, and probably some evaluation time
too. Microbenchmarking on Python 3 shows that 'is' is about 30% faster
than '==' for comparing integers on my machine.

This is safe even without assuming Python's small-integer optimization
(which caches objects for small integers, guaranteeing that small
integer literals always refer to the same object): We always refer to
symbolic constants using their symbolic names (_T_*, etc.), so they're
guaranteed to always refer to the same integer objects anyway.

This optimization is safe for client code too, unless you get some
unlikely situation where (1) there is no small-integer optimization, (2)
expressions get pickled and unpickled or the like, which would be unsafe
across Kconfiglib versions anyway, and (3) the client code tests
symbolic constants with 'is' instead of '==' (which isn't advertised as
safe, and a bad idea in general when pickling is involved). That's
probably too obscure to worry about.

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