[boost] [contract] first attempt to N3351 concepts

Hello all, I made a first attempt to sketch how N3351-like concepts will look into Boost.Contract (just for the find algorithm and its concepts for now): http://contractpp.svn.sourceforge.net/viewvc/contractpp/branches/stl_concept... This code doesn't compile quite yet but I played with its bits and pieces enough to conclude that it should be implementable as listed. It will take quite a bit of time and effort to implement this... Please tell me what you think now so I don't waste time implementing this design if it has issues. Thanks a lot! --Lorenzo