A structure for the spec of the hashing function to use for hash buckets.
hasher is the name of the hashing function to use (eg. "fasthash",
key is optional and specify the key to use for the hash function if
supported, currently only used by a strong hash.
hasher: The hasher name to use.
key: The key to be used by the hashing function, if required.
__new__( _cls, hasher, key )
Create new instance of HasherSpec(hasher, key)