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

[Raymond Hettinger]

Raymond Hettinger added the comment:

I would rather drop it than see that mess.

FWIW, there is a difference.  Rational(Decimal(...)) takes place 
without reference to a decimal.Context and is always lossless.  

In contrast, Decimal(Rational(...)) is context sensitive (the division 
is subject to rounding and precision limits) and is typically lossy as 
would be the case with Decimal(Rational(1, 3)) which like most 
rationals cannot be exactly represented in Decimal.

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