Add copy and free methods to 'struct ssh_hash'.

Add copy and free methods to 'struct ssh_hash'.

This permits a hash state to be cloned in the middle of being used, so
that multiple strings with the same prefix can be hashed without
having to repeat all the computation over the prefix.

Having done that, we'll also sometimes need to free a hash state that
we aren't generating actual hash output from, so we need a free method
as well.