Get the value and increment it by i

Property definitions

pthreads $ AtomicInt :: get_and_increment_by
	# Get the value and increment it by `i`
	fun get_and_increment_by(i: Int): Int `{
		return __sync_fetch_and_add(self, i);
	`}
lib/pthreads/pthreads.nit:92,2--95,3