[Patch] First bits of the algo merge

[Paolo Carlini]

Howard Hinnant wrote:

> I've removed _SameTypeConcept, and added the reverse 
> _BinaryPredicateConcept (which could be made neater with typedefs).  
> I think all of the set_* animals need that binary predicate both ways.

Yes, similarly to what happens already in equal_range and binary_search,
for instance. And it's certainly weaker than the current checks
therefore we cannot break any user code.

Paolo.