Rollup merge of #81048 - yoshuawuyts:stabilize-core-slice-fill-with, r=m-ou-se

Stabilize `core::slice::fill_with`

_Tracking issue: https://github.com/rust-lang/rust/issues/79221_

This stabilizes the `slice_fill_with` feature for Rust 1.51, following the stabilization of `slice_fill` in 1.50. This was requested by libs team members in #79213.

This PR also adds the "memset" alias for `slice::fill_with`, mirroring the alias set on the `slice::fill` sibling API. This will ensure someone looking for "memset" will find both variants.

r? `@Amanieu`

Author: jonas-schievink

library/core/src/slice/mod.rs

@@ -2852,13 +2852,12 @@ impl<T> [T] {
     /// # Examples
     ///
     /// ```
-    /// #![feature(slice_fill_with)]
-    ///
     /// let mut buf = vec![1; 10];
     /// buf.fill_with(Default::default);
     /// assert_eq!(buf, vec![0; 10]);
     /// ```
-    #[unstable(feature = "slice_fill_with", issue = "79221")]
+    #[doc(alias = "memset")]
+    #[stable(feature = "slice_fill_with", since = "1.51.0")]
     pub fn fill_with<F>(&mut self, mut f: F)
     where
         F: FnMut() -> T,