[issue21955] ceval.c: implement fast path for integers with a single digit

[Yury Selivanov]

Yury Selivanov added the comment:

> ### regex_v8 ###
> Min: 0.041323 -> 0.048099: 1.16x slower
> Avg: 0.041624 -> 0.049318: 1.18x slower

I think this is a random fluctuation, that benchmark (and re lib) doesn't use the operators too much.  It can't be THAT slower just because of optimizing a few binops.

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<http://bugs.python.org/issue21955>
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