Increment by i and get the new value

Property definitions

pthreads $ AtomicInt :: increment_by_and_get
	# Increment by `i` and get the new value
	fun increment_by_and_get(i: Int): Int `{
		return __sync_add_and_fetch(self, i);
	`}
lib/pthreads/pthreads.nit:112,2--115,3