Auto merge of #108476 - saethlin:remove-library-rustc-box, r=thomcc

Remove or document uses of #[rustc_box] in library

r? `@thomcc`

Only one of these uses is tested for in the rustc-perf benchmark suite. The impact there on compile time is somewhat dramatic, but I am inclined to make this change as a simplification to the library and wait for people to complain if it explodes their compilation time. I think in the absence of data or reports from users about what code paths really matter, if we are optimizing for compilation time, it's hard to argue against using `#[rustc_box]` everywhere we currently call `Box::new`.

Author: bors