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

[Jeffrey Yasskin]

Jeffrey Yasskin added the comment:

Re convergents: In the interest of minimality, I don't think
from_continued_fraction and to_continued_fraction need to stick around.
 I think the other thread established pretty conclusively that
.convergents() is not a particularly good building block for either
nearby-fraction method, so I'd let people who want it implement it
themselves. Neal Norwitz suggested and I agree that .trim() isn't
descriptive enough, so let's follow Raymond's alternative of
.limit_denominator(). Would you like to commit your implementation?

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