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

[Nick Coghlan]

Nick Coghlan added the comment:

Thanks for adding the class methods back Mark.

On the constructor front, we got a fair payoff in the early Decimal days
just by reordering the type checks in the constructor. Other than that,
I'd have to dig into the code a bit more to offer any useful suggestions.

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