Auto merge of #142728 - kornelski:string-track, r=tgross35

Let String pass #[track_caller] to its Vec calls

I've added `#[track_caller]` to `String` methods that delegate to `Vec` methods that already have `#[track_caller]`.

I've also added `#[track_caller]` to methods that have `assert!` or `panic!` due to invalid inputs.

Author: bors