Set ops in std.algorithm

[bearophile]

This post is mostly for Andrei, but I don't think private emails are good for this.
I suggest to move the set-related functions of std.algorithm to a different module, because:

1) They are tied to a specific representation of sets (sorted unique InputRanges), so if I have a BitSet, HashSet, ApproximateBloomSet, etc, they don't work (they can work with a SearchTreeSet).

2) I think there are too many names/functions in std.algorithm, and their purpose is a bit too much mixed (and currently algorithm.d module is 173 kB).

I don't know what can be a good name for such new module, sorted_input_range_set is (I think) descriptive, but a bit too much long :-)

Bye,
bearophile