class Async::Deadline

@returns [Deadline | Nil] A deadline instance, Zero singleton, or nil.
@parameter timeout [Numeric | Nil] The timeout duration, or nil for no timeout.
Create a deadline for the given timeout.

def self.start(timeout)
	if timeout.nil?
		nil
	elsif timeout <= 0
		Zero
	else
		self.new(timeout)
	end
end