Add HKDF SHA-512

Add HKDF SHA-512

In principle, we can imitate HKDF quite easily with SHA-512 alone.
Being fully RFC compliant however is fiddly and tedious, so for users
who want to do key derivation with SHA-512 in the most standard way
possible need dedicated functions.

Note the absence of a `crypto_sha512_hkdf_extract()` function, which
would be nothing more than an alias of `crypto_sha512_hmac()`.  Aliases
to the equivalent incremental interface are also absent.  There are pros
and cons to both the presence and absence of those aliases, I personally
prefer to leave them out.

See #255