Constify Extend and FromIterator

[Randl]

This is part of an attempt to fully constify Iterator. Specifically, I'm extracting the changes from the local branch that contains more or less minimal Iterator constification.

[rustbot]

r? @JohnTitor

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[JohnTitor]

I think this should also be constified, right?

View changes since the review

[Randl]

It is? It's inside const trait.

[JohnTitor]

Though the current doesn't allow to call iter.into_iter() which is less usable. I think we should also make a change like extend(), no?

[Randl]

Ah, you mean const impl? Yeah, makes sense

[Randl]

I've added the same bounds and constified impl for tuples.

[Randl]

ping @JohnTitor

[tgross35]

@rustbot blocked

Same as the others, #155816