Add checkup in tail function and return Option type

[GuillaumeGomez]

Fixes#24141.

[rust-highfive]

r? @nikomatsakis

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[lilyball]

This makes tail() inconsistent with tail_mut(), init(), and init_mut().

[lilyball]

I'm working on a separate PR that implements my proposal, of replacing tail() / init() with pop_first() / pop_last().

[lilyball]

My PR has been submitted as #24184.

[nikomatsakis]

Hmm. @kballard's PR is a bit more ambitious, so they're not directly comparable; but at minimum I agree that having tail be inconsistent with the other fns doesn't seem quite right!

[nikomatsakis]

So I'm going to leave this PR open as an alternative to #24184. I figure we might decide to take this PR, #24184, or neither. But let's move the conversation to #24141 so it can be consolidated.

[bors]

☔ The latest upstream changes (presumably #24674) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

I think I'm going to close this due to rust-lang/rfcs#1058 -- let's see how that pans out, but that approach in some form seems to be popular.

[nikomatsakis]

Thanks @GuillaumeGomez for the PR in any case!