Add test for basic custom struct

tests/ok/custom-struct.ac

@@ -0,0 +1,16 @@
+struct {
+
+struct And<P, Q>(p: P, q: Q) {
+	left = Fn<>() -> P {
+		return p;
+	};
+	right = Fn<>() -> Q {
+		return q;
+	};
+};
+
+AndLeft = Fn<P, Q>(h: And<P, Q>) -> P {
+	return h.p;
+};
+
+} main;