Fix compilation warnings (GCC 4.7)

This commit does the follwing:
- fixes a bunch of "const" declaration missing
- set static a declaration inside a .h file (partial_precompute_tworounds) to prevent missing declaration warnings
- add the declaration of crypto_onetimeauth_poly1305_implementation_name which was missing (remove the declaration and set static if it should not be exposed)
- modifies the definition of crypto_stream_aes256estream_afternm and crypto_stream_aes256estream_xor_afternm to fix a couple of "unconstified" casts which i couldn't fix otherwise
- comments the stuff in compat.c which seems to be unused and, for that reason, is missing functiond declarations.