[v3] Remove algos code duplication

[Paolo Carlini]

Hi,
>     Maybe it is simply time to merge what has been done by Howard and 
> Paolo in libstdcxx_so_7-branch. I will take a closer look to it and 
> hope that Howard will tell us if there is anything wrong in merging 
> this work now.
Realistically, I don't think Howard can provide further guidance today, 
give his strong involvement with clang. I mentioned him just to make 
sure people have all the "keywords" to find the relevant old exchanges 
in the mailing list.

I suppose we want indeed to review the work that Chris did (with some 
inputs from Howard) and recheck that it's still *correct* and *as simple 
as possible* today, given real rvalue references, etc. In terms of 
testing probably we want either to add a proxy iterator to the testsuite 
infrastructure and double check that we don't regress when we 
instantiate all the algorithms with it, or just use 
vector<bool>::iterator for that. I'm not sure the latter is nasty enough 
(as proxy iterator). Maybe Daniel too (in CC) can provide some help 
about this.

Thanks,
Paolo.