Rollup merge of #79213 - yoshuawuyts:stabilize-slice-fill, r=m-ou-se

Rollup merge of #79213 - yoshuawuyts:stabilize-slice-fill, r=m-ou-se

Stabilize `core::slice::fill`

Tracking issue https://github.com/rust-lang/rust/issues/70758

Stabilizes the `core::slice::fill` API in Rust 1.50, adding a `memset` doc alias so people coming from C/C++ looking for this operation can find it in the docs. This API hasn't seen any changes since we changed the signature in https://github.com/rust-lang/rust/pull/71165/, and it seems like the right time to propose stabilization. Thanks!

r? `@m-ou-se`