Hash Functions
A hash function is essentially an 
std::unary_operator. It takes a
key_type_defs.htmlconst_key_reference , and returns
a (non-negative) integral type.
	A special hash function, 
null_hash_fn.htmlnull_hash_fn serves to indicate that the 
hash_policies.htmlranged-hash  function
or 
hash_policies.htmlranged-probe  function should not be formed by composition.
