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

[Yury Selivanov]

Yury Selivanov added the comment:

> Fast patch is already implemented in long_mul(). May be we should just use this function if both arguments are exact int, and apply the switch optimization inside.

Agree.

BTW, what do you think about using __int128 when available?  That way we can also optimize twodigit PyLongs.

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