chacha_vec.c; updates API, fixes for correctness issues:

1. 64-bit, defined(__ARM_NEON__): on both codepaths, x13 always equal to zero
2. defined(__ARM_NEON_): on the GPR codepath, x13 is never incremented within a call to Chacha (even if, e.g., counter == 2**32-1 at entry)