compiler: add generics test case

This test case shows a few bugs, that I will fix in subsequent patches.

compiler/testdata/generics.go

@@ -0,0 +1,21 @@
+package main
+
+type Coord interface {
+	int | float32
+}
+
+type Point[T Coord] struct {
+	X, Y T
+}
+
+func Add[T Coord](a, b Point[T]) Point[T] {
+	return Point[T]{
+		X: a.X + b.X,
+		Y: a.Y + b.Y,
+	}
+}
+
+func main() {
+	var af, bf Point[float32]
+	Add(af, bf)
+}