Add support for the Skein family of hash functions.
Skein uses the Davies-Meyer construction, which increases the overall variety of hash functions supported by rhash. MD4, MD5, SHA1, and SHA2 all use the Merkle–Damgård construction, whirlpool uses the Miyaguchi-Preneel construction, and SHA3 uses the sponge construction.
Skein supports internal state sizes of 256, 512 and 1024 bits, and arbitrary output sizes. It's one of the fastest modern cryptographic hashes, claiming 6.1 cycles per byte for any output size on an Intel Core 2 Duo in 64-bit mode, was a candidate for the SHA3 NIST competition, and is believed currently to be secure.
See https://en.wikipedia.org/wiki/Skein_(hash_function) for more info.