go/neo/proto: protogen: Reuse sizer for typeSizeFixed

Generating code through sizer can already give answer if a type is fixed
size or not. And in practice we don't care at all if it will be a bit
slower compared to previous explicit type switch.

For current code it is just a small cleanup, and it will be handy to
support messagepack encoding as well.

No change in generated code.

go/neo/proto/protogen.go

@@ -424,34 +424,13 @@ var basicTypes = map[types.BasicKind]basicCodec{
 
 // does a type have fixed wire size and, if yes, what it is?
 func typeSizeFixed(typ types.Type) (wireSize int, ok bool) {
-	switch u := typ.Underlying().(type) {
-	case *types.Basic:
-		basic, ok := basicTypes[u.Kind()]
-		if ok {
-			return basic.wireSize, ok
-		}
-
-	case *types.Struct:
-		for i := 0; i < u.NumFields(); i++ {
-			size, ok := typeSizeFixed(u.Field(i).Type())
-			if !ok {
-				goto notfixed
-			}
-			wireSize += size
-		}
-
-		return wireSize, true
-
-	case *types.Array:
-		elemSize, ok := typeSizeFixed(u.Elem())
-		if ok {
-			return int(u.Len()) * elemSize, ok
-		}
+	// pass typ through sizer and see if encoded size is fixed or not
+	s := &sizer{}
+	codegenType("x", typ, nil, s)
+	if !s.size.IsNumeric() { // no symbolic part
+		return 0, false
 	}
-
-notfixed:
-	// everything else is of not fixed wire size
-	return 0, false
+	return s.size.num, true
 }
 
 // interface of a codegenerator  (for sizer/coder/decoder)