Following on from the review for D58088, this patch provides the
prerequisite to_address() implementation that's needed to have
pointer_iterator support unique_ptr.
'typename Ptr::element_type *' should be replaced with 'auto' once we move
to C++14 to better align with the C++20 declaration. Until then, this
implementation is limited to fancy pointers with an element_type typedef.
Also, this implementation can be removed once we move to C++20 where it's
defined as std::to_addres()
The std::pointer_traits<>::to_address(p) variations of these overloads has
not been implemented.