Remove algo duplication

[Paolo Carlini]

Hi,

as far as I'm concerned, the sheer size of predefined_ops.h is the most important issue. I still can't convince myself on an intuitive level that for forwarding so much code is necessary.

That said, since the real difficulty of the task boils down to proper fowarding, it seems "obvious" to me that a complete patch should come with corresponding tests, minimally exercizing the algorithms for a typical proxy iterator type, maybe even the available vector<bool>::iterator, or a specific new test type (more "nasty" than vector<bool>::iterator? In the past Howard often discussed some, in various contexts). I realize it would be a lot of work and in any case I can't promise that I would immediately approve the patch (for the reason above)

Paolo