[Bug libstdc++/24693] Deque improvements

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24693

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Prototype patch: https://gcc.gnu.org/ml/libstdc++/2016-10/msg00017.html