hkdf: HMAC-based Extract-and-Expand Key Derivation Function

This module provides a HKDF implementation according to RFC 5869. A pesudorandom
key is created using [[extract]], which in turn is used to derive keys using
[[expand]]. [[hkdf]] combines both [[extract]] and [[expand]] for deriving
a single key.

See the RFC 5869 for detailed usage guidance on how to choose the parameters.
