feature: golang-style `defer` for setting `OnExit`

Can we add a golang-`defer`-like command that sets `OnExit`?

E.g. consider the case of an application that has bound an HTTP path. If that application dies, the path remains bound, since the application did not call unbind.

However, this is a natural case for OnExit-based cleanup; and possibly for a `defer`-like call.