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`

Author: bors