A continuation that captures the current Async execution context.
It can be scheduled for future invocation by installing it in a deferred.
val create : ('a ‑> unit) ‑> 'a tcreate 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.
filter h ~f makes the eventual execution of the handler h on a value v dependent
on whether predicate f holds of v
val install : 'a t ‑> 'a Async_kernel.Deferred.t ‑> unit ‑> unitinstall h d behaves like upon except that it also returns a uninstall function
that, when called, uninstalls the handler
val schedule : 'a t ‑> 'a ‑> unitschedule h v schedules the handler h to run at some point in the future by being
called on value v