Tested on Linux-x64.

2016-11-13  Ville Voutilainen  <ville.voutilainen@gmail.com>

    Implement P0513R0, Poisoning the Hash.
    * include/bits/functional_hash.h (__poison_hash): New.
    * include/bits/unique_ptr.h
    (hash<unique_ptr<_Tp, _Dp>>): Derive from __poison_hash.
    * include/std/optional (hash<optional<_Tp>>): Likewise.
    * include/std/variant (hash<variant<_Types...>>): Likewise.
    * testsuite/20_util/optional/hash.cc: New.
    * testsuite/20_util/unique_ptr/hash/1.cc: Add tests for
    poisoned fancy pointer hashes.
    * testsuite/20_util/variant/hash.cc: New.