Add trim view

[JohelEGP]

I hope this is within scope. I thought I'd try getting this in so that it's available for the broader public rather than stuffing it in a library of mine. I'm looking forward to the review comments.

The current implementation is simple thanks to the bidirectional range requirement. Looking at #1112, it should be possible to do with a forward range. But I believe it'd be simpler to add a complementing drop_last_while and implement view::trim in terms of it and drop_while. This possibility makes me wonder if this is okay as is.

[ericniebler]

You have certainly done your homework here. Nice work. Just a few small changes. I'm not convinced this can be implemented for non-Bidirectional ranges.

[JohelEGP]

The latter part of d28764f's message shouldn't matter as the range concepts assign no semantics at all to default initialization.

[JohelEGP]

I guess default-initialized views are to be treated as the moved-from state (like being unable to get their begin/end). I have reverted the attempt to assert whether pred_ was not in its default-initialized state.

[JohelEGP]

Is it alright to mark find_if_not as constexpr /*c++14*/ so that trim_view can be constexpr too? I thought that you were holding back the algorithms from being constexpr since their standard version weren't, but I noticed that equal is constexpr.

[ericniebler]

Is it alright to mark find_if_not as constexpr /*c++14*/ so that trim_view can be constexpr too? I thought that you were holding back the algorithms from being constexpr since their standard version weren't, but I noticed that equal is constexpr.

Fine by me.