handy.h: isASCII() extend to work on > 8 bit values

handy.h: isASCII() extend to work on > 8 bit values

Prior to this patch, if isASCII() is called with something like '256',
it would return true.

For some reason unknown to me, U64 is defined only inside the perl core.
However, the equivalent U64TYPE is known everywhere, so in the macro
that can be called outside of core, use that instead.

The commit log doesn't give a reason for not defining U64 outside of
core, and no tests in the suite fail when it is defined outside core.
But out of caution, I'm just doing this workaround instead of exposing
U64.