Rollup merge of #56548 - Lucretiel:string-extend-optimize, r=sfackler

Rollup merge of #56548 - Lucretiel:string-extend-optimize, r=sfackler

Optimized string FromIterator + Extend impls

I noticed that there was a lost opportunity to reuse string buffers in `FromIterator<String>` and `FromIterator<Cow<str>>`; updated the implementations to use these. In practice this translates to at least one fewer allocation when using these APIs.

Additionally, rewrote `Extend` implementations to use `iter.for_each`, which (supposedly) helps the compiler optimize those loops (because iterator adapters are encouraged to provide optimized implementations of `fold` and `try_fold`.