* Generalize the type of `runRW#` to allow arbitrary return types.

* Use `runRW#` to implement `Control.Monad.ST.Lazy.runST` (this
provides evidence that it actually works properly with the generalized
type).

* Adjust the type signature in the definition of `oneShot` to match
the one it is given in `MkId`.

Reviewers: simonmar, austin, bgamari, hvr

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3012

GHC Trac Issues: #13178