Renamed "crypto_hmac_*" to "crypto_hmac_sha512_*"

Renamed "crypto_hmac_*" to "crypto_hmac_sha512_*"

There are several types of HMAC, and users may want to use other
versions of HMAC as well. (For instance, they could code their own
Blake2b HMAC to implement Noise).  Plus, most primitives are named by
their technical name. "hmac" alone is not enough.

The names are longer, but this is the optional part, after all.