Pin stabilization

[cramertj]

This implements the changes suggested in #55766 (comment) and stabilizes the pin feature. @alexcrichton also listed several "blockers" in that issue, but then in this comment mentioned that they're more "TODO items":

In that vein I think it's fine for a stabilization PR to be posted at any time now with FCP lapsed for a week or so now. The final points about self/pin/pinned can be briefly discussed there (if even necessary, they could be left as the proposal above).

Let's settle these last bits here and get this thing stabilized! :)

r? @alexcrichton
cc @withoutboats