Proposed amendment to SE-202: Replace `Collection.randomElement` requirement with `randomIndex`

[Joe_Groff]

@beccadax

Don't other members like first have the same problem?

I raised the issue with first in another thread. @Ben_Cohen said it's a unnecessary customization point he can remove.

@beccadax

Should we address this in the moveonly design by allowing a protocol to say "this requirement only applies to non- moveonly conformances"? I've wanted a similar feature for other purposes—particularly, I've wanted to be able to put a func index(of elem: Element) -> Index? where Element: Equatable requirement so we could replace the private _customIndexOfEquatableElement(_:) requirement with something publicly usable.

@Douglas_Gregor has expressed concerns about implementing such a feature. All things equal, it'd be better to avoid tying the move-only feature to more prerequisite features if we can avoid it.