[Patch, v7] Prepare for DR 431 everywhere

[Paolo Carlini]

Hi,

the below is all the infrastructure I need to implement very simply
Option 3 for all the containers (+ string): just matter of begin able to
access the allocator by reference.

As hoped at the outset, I'm now sure that we can do that without
breaking binary compatibility: after completing the work next week we
can test it for a while in the branch and then merge it to mainline.

Also took the occasion to fix a very-very stupid bug in the
uneq_allocator testing class, which showed up while prototyping Option 3
for vector.

Tested x86-linux.

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