[issue1682] Move Demo/classes/Rat.py to Lib/fractions.py and fix it up.

[Jeffrey Yasskin]

Jeffrey Yasskin added the comment:

Thanks for writing the __add__ optimization down. I hadn't realized how
simple it was. I think both optimizations are worth it. 22% on a rarely
used class is worth 24 lines of python, and I think nearly eliminating
the overhead of ABCs (which will prevent them from getting a bad
performance reputation) is worth some C.

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