A continuation that captures the current Async execution context.
It can be scheduled for future invocation by installing it in a deferred.
create k creates a handler by coupling the continuation k together with the
evaluation context that is current at the time create is called. Whenever this
handler is later invoked, it will happen in this saved evaluation context.
install h d behaves like upon except that it also returns a uninstall function
that, when called, uninstalls the handler
schedule h v schedules the handler h to run at some point in the future by being
called on value v