…compile time in WASM

This is not unsafe on WASM, see rust-lang#84988

…, r=Mark-Simulacrum

Implement `OnceCell/Lock::try_insert()`

I took inspiration from [`once_cell`](https://crates.io/crates/once_cell):
- [`once_cell::unsync::OnceCell::try_insert()`](https://github.com/matklad/once_cell/blob/874f9373abd7feaf923a3b3c34bfb3383529c671/src/lib.rs#L551-L563)
- [`once_cell::sync::OnceCell::try_insert()`](https://github.com/matklad/once_cell/blob/874f9373abd7feaf923a3b3c34bfb3383529c671/src/lib.rs#L1080-L1087)

I tried to change as little code as possible in the first commit and applied some obvious optimizations in the second one.

ACP: rust-lang/libs-team#276
Tracking issue: rust-lang#116693

…eatures, r=RalfJung

const-eval: allow calling functions with targat features disabled at compile time in WASM

This is not unsafe on WASM, see rust-lang#84988

r? `@RalfJung`

Fixes rust-lang#116516

…r-state, r=Mark-Simulacrum

Make "request changes" reviews apply `S-waiting-on-author`

This makes it so **assignee** requesting changes on a PR via GitHub UI adds https://github.com/rust-lang/rust/labels/S-waiting-on-author and removes https://github.com/rust-lang/rust/labels/S-waiting-on-review.

cc `@compiler-errors` :3