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.

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

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[JohnTitor]

I think this should also be constified, right?

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[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.