[Patch, v7] DR 431 for sso/rc-strings

[Paolo Carlini]

Paolo Carlini wrote:

>Most of this first part actually consisted in figuring out a
>sufficiently flexible and reliable testing allocator, uneq_allocator,
>which hopefully we can keep on using for string and for the containers.
>  
>
Forgot to thank publically Howard for reassuring me that a first draft
of the class was not completely off and for advices on a couple of very
tricky details.

Paolo.