all: implement ch <- x or {...} and ch <- x ? (#7928)

vlib/sync/channel_push_or_2_test.v

@@ -0,0 +1,27 @@
+const n = 1000
+
+fn f(ch chan int) {
+	mut s := 0
+	for _ in 0 .. n {
+		s += <-ch
+	}
+	assert s == n * (n + 1) / 2
+	ch.close()
+}
+
+fn do_send(ch chan int, val int) ?int {
+	ch <- val ?
+	return val + 1
+}
+
+fn test_push_propargate() {
+	ch := chan int{}
+	go f(ch)
+	mut s := 1
+	for {
+		s = do_send(ch, s) or {
+			break
+		}
+	}
+	assert s == n + 1
+}