Emulate some C99 macros and typedefs

Emulate some C99 macros and typedefs

On systems that don't have them, this emulates

U?INT(16|32)_C
U?INTMAX_C

and the typedefs

u?intmax_t

Since, these are typedefs that can't be tested for if they exist, this
creates PERL_U?INTMAX_T typedefs instead, setting them to the standard
values in stdint.h if it is available.

In addition, it moves the pre-existing emulation of U?INT64_C from
handy.h to perl.h.  This is because there was duplicate-ish logic in the
two files, and the handy.h version appears to be better thought out.

It converts the couple of places in core that were using the other
deleted logic to instead use the C99 names.