This is a proof-of-concept showing how we can fix some known
deficiencies in std::deque (24693, 77524, throwing moves) for builds
using --enable-symvers=gnu-versioned-namespace, which don't need to
preserve ABI compatibility.

I'm also considering defining an allocator adaptor that would also
enable these fixes, so that std::deque<T, __fixed_deque<A>> would
be equivalent to std::deque<T, A> except that it would enable these
improvements even for the default --enable-symvers=gnu mode.

There should be very little overhead to the extra code, because most
of the new branches depend on compile-time constants.

Are people interested in seeing this for GCC 7?