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.
schedule h v schedules the handler
h to run at some point in the future by being
called on value