libstdc++: [_Hashtable] Use next bucket node and equal_to to check if 
same bucket

To find out if we are still in the same bucket we can first check that 
current node
is not the next bucket's before-begin and then that hash code are equals 
when cached.
If not we can also use the equal_to functor in a multi-container 
context. As a last
resort, compute node bucket index.

libstdc++-v3/ChangeLog:

     * include/bits/hashtable_policy.h 
(_Hashtable_base<>::_S_hash_code_equals): New.
     * include/bits/hashtable.h (_Hashtable<>::_M_is_in_bucket): New, 
use latter.
     (_Hashtable<>::_M_find_before_node): Use latter.
     (_Hashtable<>::_M_find_before_node_tr): Likewise.

Tested under Linux x86_64.

François