[Patch] Implement Option 3 of DR 431

[Paolo Carlini]

Howard Hinnant wrote:
> Caution for mainline:  C++03 allocators are not required to be 
> swappable (they have copyconstructible but lack copyassignable).  In 
> practice this doesn't appear to be a major problem, and can be easily 
> fixed by adding a swap to your non-assignable allocator.  Just thought 
> I'd better mention it.
I know about that, of course, and we already discussed the issue a bit. 
Given that otherwise container::swap is totally broken for unequal 
allocators, and your analysis in N1599, it's the best we can do... well 
you agree of course ;) and Gaby too. Maybe, additionally to mentioning 
DR 431 in the docs, as we usually do by default for implemented DRs, we 
can also add a note to the 4.2 release notes, but we have got time for 
that. If nobody objects, I'm definitely going to commit this work to 
mainline too.

Paolo.