integer parser fixed/replaced thanks to cluts

[Rich Felker <dalias@aerifal.cx>]

Thanks to cluts' numeric tests, I've tracked down the errors in musl's
integer parsing routines and rewritten them as wrappers around a
unified core parser that can handle char-by-char or whole-string input
(or any mix). Eventually scanf will also use the new parser, reducing
code size and most likely improving correctness.

I also plan to write a new floating point parser based on the same
principles, which will facilitate adding the currently-missing
wcstof/wcstod/wcstold functions, and at the same time I hope to
address inexactness issues.

Rich