engine, lang: Allow resources with a field of type interface

This lets us add a resource that has an implementation with a field
whose type is determined at compile time. This let's us write more
flexible resources.

What's missing is additional type checking so that we guarantee that a
specific resource doesn't change types during run-time.

lang/types/type.go

@@ -98,6 +98,7 @@ func ResTypeOf(t reflect.Type) (*Type, error) {
 		StructTagOpt(StructTag),
 		StrictStructTagOpt(true),
 		SkipBadStructFieldsOpt(true),
+		AllowInterfaceTypeOpt(true),
 	}
 	return ConfigurableTypeOf(t, opts...)
 }

lang/types/value.go

@@ -283,6 +283,15 @@ func Into(v Value, rv reflect.Value) error {
 	if typ == nil {
 		return fmt.Errorf("cannot Into() %+v of type %s into a nil type", v, v.Type())
 	}
+	// This is used when we are setting a resource field which has type of
+	// interface{} instead of a string, bool, list, etc...
+	if isInterface := typ.Kind() == reflect.Interface; isInterface {
+		//x := reflect.ValueOf(v) // no!
+		// use the value with type interface{}, not types.Value
+		x := reflect.ValueOf(v.Value())
+		rv.Set(x)
+		return nil
+	}
 
 	switch v := v.(type) {
 	case *BoolValue: