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`.