Tell actor to terminate now

Queueing a ShutDownMessage before every other ones

Property definitions

actors $ Proxy :: terminate_now
	# Tell `actor` to terminate now
	# Queueing a ShutDownMessage before every other ones
	fun terminate_now do
		var msg = new ShutDownMessage
		actor.mailbox.unshift(msg)
	end
lib/actors/actors.nit:151,2--156,4