Code quality improvements

[msprotz]

While trying to minimize the generated C code for @NikolajBjorner:

• too many files include libintvector.h indiscriminately
• evercrypt_targetconfig.h, ibid.
• kremlib (minimal) depends on compat.h which is nonsensical -- all of the stuff that mentions Prims from the machine integers should just be marked as noextract
• cruft shipped in the kremlib directories: Makefile.{basic,include} and .def file
• lots of stuff in types.h that should go in "full" kremlib, or maybe not even be there except for bootstrapping?
• purpose of KRML_HOST_SNPRINTF?
• excessive inclusion of Hacl_Kremlib.h in numerous files... spurious?
• too many unused variable errors -- maintaining a list in Helpers.ml that's hardcoded is not scalable, can we add an F* attribute somewhere for assumed definitions that in addition to assuming their types, assume that they are pure for the sake of triggering kremlin's unused variable elimination?

[msprotz]

Ongoing work for unused variable elimination going on in #582 (it's tricky, so need to iterate before we have something that works just fine)