Auto merge of #50017 - tinaun:stabilize-all-the-things, r=sfackler

Auto merge of #50017 - tinaun:stabilize-all-the-things, r=sfackler

stabilize a bunch of minor api additions

besides `ptr::NonNull::cast` (which is 4 days away from end of FCP) all of these have been finished with FCP for a few weeks now with minimal issues raised

* Closes #41020
* Closes #42818
* Closes #44030
* Closes #44400
* Closes #46507
* Closes #47653
* Closes #46344

the following functions will be stabilized in 1.27:
* `[T]::rsplit`
* `[T]::rsplit_mut`
* `[T]::swap_with_slice`
* `ptr::swap_nonoverlapping`
* `NonNull::cast`
* `Duration::from_micros`
* `Duration::from_nanos`
* `Duration::subsec_millis`
* `Duration::subsec_micros`
* `HashMap::remove_entry`