Delete `caffeine.h`

[ktras]

caffeine.h is not necessary since we do not want to make the caf_ procedures available.

• [x] Delete the file around the time that the module restructuring occurs (issue #64).
• [x] Audit function namespace (Make things either static of ensure has caf_ prefix as needed)
• [x] Rename as_int -> caf_as_int, etc (at least one more)
• [x] Make set_stat_errmsg_or_abort, static
• [x] Add function namespace policy to README-maintainers.md