Extract the hash and equality functions from hash_table_data_*.
As these are extensions and add extra requirements to the container elements,
they shouldn't be part of hash_table_data_* so that they won't get instantiated
if an unordered container is explicitly instantiated.
Merged revisions 46594-46604 via svnmerge from
https://svn.boost.org/svn/boost/branches/unordered/trunk