Rollup merge of #85835 - Seppel3210:master, r=yaahc

Rollup merge of #85835 - Seppel3210:master, r=yaahc

Implement Extend<(A, B)> for (Extend<A>, Extend<B>)

I oriented myself at the implementation of `Iterator::unzip` and also rewrote the impl in terms of `(A, B)::extend` after that.

Since (A, B) now also implements Extend we could also mention in the documentation of unzip that it can do "nested unzipping" (you could unzip `Iterator<Item=(A, (B, C))>` into `(Vec<A>, (Vec<B>, Vec<C>))` for example) but I'm not sure of that so I'm asking here 🙂

(P.S. I saw a couple of people asking if there is an unzip3 but there isn't. So this could be a way to get equivalent functionality)