LWG Poll 13: P1994R1 elements_view needs its own sentinel

[burblebee]

Also fixes LWG3386.

Fixes #3715.
Fixes cplusplus/papers#717.

Notes/Issues:

• [range.elements.sentinel] Should \exposid{Base} be \exposid{base-t} as it is in [range.elements.iterator]? We're inconsistent wrt Base v. base-t elsewhere as well. There are also other differences between sentinel and iterator, like "other" and "base() const" (no ref) - I assume these are intentional?

[CaseyCarter]

• [range.elements.sentinel] Should \exposid{Base} be \exposid{base-t} as it is in [range.elements.iterator]? We're inconsistent wrt Base v. base-t elsewhere as well.

Per #3561, both should be \exposid{Base}.

There are also other differences between sentinel and iterator, like "other" and "base() const" (no ref) - I assume these are intentional?

Yes they are: iterator may adapt a move-only iterator, but sentinels are always semiregular and therefore copyable.