Prefer intmax_t to int64_t in module code

Prefer intmax_t to int64_t in module code

* modules/mod-test/mod-test.c (sum, Fmod_test_sum):
* src/emacs-module.c (module_extract_integer)
(module_make_integer):
* src/emacs-module.h (struct emacs_env_25):
Prefer intmax_t to int64_t.  This doesn’t change the generated
code on any of the machines Emacs currently ports to, but it’s
at least in theory more future-proof as C99 doesn’t guarantee
that int64_t exists.