SHAKE256 extendable output function implementation

SHAKE256 is a extendable output function defined in the SHA-3 standard. When instantiated correctly, SHAKE256 is an acceptable hash function for some purposes. Notably the Ed448 signature scheme uses SHAKE256 to produce a message digest.

SHAKE256 should be implemented in order to implement Ed448. It is not necessary to expose SHAKE256 as a MessageDigest, and the Ed448 implementation in SunEC will use a private API to invoke SHAKE256. It is also not necessary to implement SHAKE128.