Rollup merge of #41449 - Eh2406:master, r=aturon
Rollup merge of #41449 - Eh2406:master, r=aturon

FromIterator and Extend Cow for String

This is a quick draft to start working on [#41351](https://github.com/rust-lang/rust/issues/41351).
I don't think I got the stable attributes correct, but it is good enuf to start a conversation.

aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust