impls. This is a [breaking-change] (for gated code) in that when you
implement `Fn` (`FnMut`) you must also implement `FnOnce`. This commit
demonstrates how to fix it.
specialized to closures, and invoke them as soon as we know the
closure kind. I thought initially we would need a fixed-point
inference algorithm but it appears I was mistaken, so we can do this.
