Implement pointer_traits::to_address as in P0653r0

        * include/bits/allocated_ptr.h (allocated_ptr): Use
pointer_traits::to_address.
        * include/bits/ptr_traits.h (pointer_traits): Implement to_address.
        * include/ext/pointer.h (pointer_traits): Define to_address in
pointer_traits specialization.
        * testsuite/20_util/pointer_traits/requirements/explicit_instantiation.cc:
Define operator->.
        * testsuite/20_util/pointer_traits/to_address.cc: New tests.

Tested i686-pc-linux-gnu.

Glen