cfunction
(ev/deadline sec &opt tocancel tocheck)
Set a deadline for a fiber `tocheck`. If `tocheck` is not finished
after `sec` seconds, `tocancel` will be canceled as with `ev/cancel`.
If `tocancel` and `tocheck` are not given, they default to
`(fiber/root)` and `(fiber/current)` respectively. Returns
`tocancel`.