caller of `Extend` should call clone

[lonnen]

Matklad suggests removing Extend implementations for Extend<(&'a T, &'a usize)> and Extend<(&'a T)> because they obscure a simple clone that the caller could explicitly do without these.

de16931#diff-b1a35a68f14e696205874893c07fd24fdb88882b47c23cc0e0c80a30c7d53759R1186

[lonnen]

I keep going back and forth on this. We need clone for compact storage-by-value but I don't remember why I needed it here.

Consider also the ultimate implementation of extend for sets in the stdlib:
https://docs.rs/hashbrown/0.1.8/src/hashbrown/map.rs.html?search=#2436