Rollup merge of #34890 - oconnor663:addassign, r=brson

Rollup merge of #34890 - oconnor663:addassign, r=brson

implement AddAssign for String

Currently `String` implements `Add` but not `AddAssign`. This PR fills in that gap.

I played around with having `AddAssign` (and `Add` and `push_str`) take `AsRef<str>` instead of `&str`, but it looks like that breaks arguments that implement `Deref<Target=str>` and not `AsRef<str>`. Comments in [`libcore/convert.rs`](https://github.com/rust-lang/rust/blob/master/src/libcore/convert.rs#L207-L213) make it sound like we could fix this with a blanket impl eventually. Does anyone know what's blocking that?