Auto merge of #126877 - GrigorenkoPV:clone_to_uninit, r=dtolnay

CloneToUninit impls

As per #126799.

Also implements it for `Wtf8` and both versions of `os_str::Slice`.

Maybe it is worth to slap `#[inline]` on some of those impls.

r? `@dtolnay`

library/std/src/lib.rs

@@ -319,6 +319,7 @@
 // tidy-alphabetical-start
 #![feature(c_str_module)]
 #![feature(char_internals)]
+#![feature(clone_to_uninit)]
 #![feature(core_intrinsics)]
 #![feature(core_io_borrowed_buf)]
 #![feature(duration_constants)]