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...)
 }