On the Rust side, this expects a typical C callback function with a
"context" parameter. On the Swift side, we pass a manually-refcounted
object as that "context" which can be used to complete a
CheckedContinuation, bridging into the language 'async' functionality.
The main obstacle to this approach is that Swift does not allow C
function pointers to be used in generic ways, due to its run-time
generics model. AsyncUtils.swift describes the workarounds needed to
deal with this.
