29a74e6285
Add `core::slice::fill_with` Tracking issue https://github.com/rust-lang/rust/issues/79221. As suggested by `@m-ou-se` in https://github.com/rust-lang/rust/issues/70758#issuecomment-726838099 this implements `slice::fill_with` as a counterpart to `slice::fill`. This mirrors `Vec::resize` and `Vec::resize_with`. Thanks! r? `@m-ou-se`