Tell actor to terminate properly

Queueing a ShutDownMessage to the end of its mailbox

Property definitions

actors $ Proxy :: terminate
	# Tell `actor` to terminate properly
	# Queueing a ShutDownMessage to the end of its mailbox
	fun terminate do
		var msg = new ShutDownMessage
		actor.mailbox.push(msg)
	end
lib/actors/actors.nit:144,2--149,4