Specify warning for internal only headers that are now installed for use
by extensions.

effects throughout, including use of inttypes.h/PRI[ud]64 because printf() is
still stuck in a long world, conversion of %lu to %zu for printing values of
type size_t, and changing/renaming the g_check_cmp* family of functions.

Refactored all of the desugaring code to no longer depend on memory being initialized to 0. Everything is about 12% faster now.