add tests for low-level 'nacl.c' API

This includes a disabled test for crypto_scalarmult() (the two-argument
function, not the one-argument crypto_scalarmult_base()), which isn't
implemented yet. It also has a test vector for test_secretbox() that I
generated with a C program linked against libnacl.a .
fix raw crypto_secretbox API
fix raw crypto_box API
fix raw crypto_sign API