[Prelim patch] Speed up ints parsing

[Loren James Rittle]

> ... and the results for sync_with_stdio(false); are even more
> impressing (the num_get component is more relevant in this
> case): [...performance numbers...]

Paolo, sweet!  What sort of functionality testing have you done?  Last
I looked, our test suite doesn't run through many corner cases on
numeric input esp. system-specific cases.  Also, there used to be a
claim that we wanted to use libc for such input to match per-system
quirks.  However, I have no strong opinions on the matter and support
your proposal in principle.

Regards,
Loren