Implement HKDF from RFC5869

This is a customizable extract-and-expand HMAC-KDF for deriving keys.
It derives from RFC5869, which derives its rationale from Krawczyk,
H., "Cryptographic Extraction and Key Derivation: The HKDF Scheme",
Proceedings of CRYPTO 2010, 2010, <http://eprint.iacr.org/2010/264>.

I'm also renaming the existing KDF, now that Tor has two of them.

This is the key derivation scheme specified in ntor.

There are also unit tests.