Fix hashtable memory leak

[François Dumont]

There a memory leak when move assigning between 2 instances with 
different bucket count and non-propagating and unequal allocator 
instances (see new test03).

I reduced code duplication to fix the issue as 1 of the 2 
implementations was fine.

 Â Â Â  * include/bits/hashtable.h (_Hashtable<>::_M_replicate): New.
 Â Â Â  (_Hashtable<>::operator=(const _Hashtable&)): Use latter.
 Â Â Â  (_Hashtable<>::_M_move_assign(_Hashtable&&, false_type)): Likewise.
 Â Â Â  * testsuite/23_containers/unordered_set/allocator/move_assign.cc
 Â Â Â  (test03): New.

I still need to run all tests but new move_assign.cc works fine.

Ok to commit once fully tested ?

François

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