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

[STINNER Victor]

STINNER Victor added the comment:

I prefer fastint_alt.patch design, it's simpler. I added a comment on the review.

My numbers, best of 5 timeit runs:

$ ./python -m timeit -s "x = 12345" -- "x*2; x*2; x*2; x*2; x*2; x*2; x*2; x*2; x*2; x*2; "

* original: 299 ns
* fastint2.patch: 282 ns (-17 ns, -6%)
* fastint_alt.patch: 267 ns (-32 ns, -11%)

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