New functions: fold_on_nonequal_inter and fold_on_nonequal_union

[mlemerre]

Was called fold_on_diff in Codex. No tests yet

[dlesbre]

I've suggested some corrections on improvements.

It would be nice, and fairly easy, to have a specialized version of these functions for non-generic maps.

Also adding tests shouldn't be too hard.

[mlemerre]

Thanks for the suggestion! I forgot to tell that the patch was not ready yet (homogeneous version and tests). I was not fully aware of the unsigned changes, so thanks for the improvements ideas

[mlemerre]

It should be good now: I have added the tests and the homogeneous version. The tests obviously revealed that I messed up with the union case (much more complicated than the intersection one).

If you find a way to improve the code, then go for it (I don't see obvious ways to do it). Also please tell me if you think the doc is fine. Then we should be able to merge!

[dlesbre]

I've replaced a remaining ma < mb with unsigned_lt, simplified g by removing extra arguments, and improved the doc a bit (multiline code blocks with [ ... ] don't render well: they keep all whitespace as is).