Auto merge of #111849 - eholk:uniquearc, r=Mark-Simulacrum

Add `alloc::rc::UniqueRc`

This PR implements `UniqueRc` as described in rust-lang/libs-team#90.

I've tried to stick to the API proposed there, incorporating the feedback from the ACP review. For now I've just implemented `UniqueRc`, but we'll want `UniqueArc` as well. I wanted to get feedback on this implementation first since the `UniqueArc` version should be mostly a copy/paste/rename job.

Author: bors