This shows I got some optimizing to do. I was expecting integer parsing
to be slower, though, but it looks like it can compete with JSON::XS's
specialized small-int parsing code anyway.
Going to need a way to pass arguments into the XS function anyway, so
might as well do the entire arg parsing step in XS while we're at it.
Provides a significant speedup for tiny inputs as well, but I don't find
that too interesting.
