Implement N4009 - Uniform container erasure (erase_if)

[gnzlbg]

N4009 - Uniform container erasure defines erase_if which seems analogous to Boost.Range v2 erase_remove_if container algorithm.

[ericniebler]

I've recently started a broad investigation into container algorithms in general, per a recent discussion with @sean-parent. I think we've recently hit upon something. erase_if is certainly a likely candidate.

[gnzlbg]

I've been completely stuck with work since two months already and that won't probably change in the foreseeable future.

I think that with erase_if your library would be a complete replacement of Boost.Range v2 already.

An interesting exercise could be to rewrite all Boost.Range v2 examples using range-v3 both as documentation and to see if something is missing.

[ericniebler]

Docs are a huge TODO, and porting the Boost.Range examples might be a good way to jump-start that effort, but I'm not worried about replacing Boost.Range in its entirety. erase_if is a container algorithm. Those are different beasts than range algorithms. Either we do container algorithms right as a class, or else it gets left out. I'm OK with that.