Restructure the generation of `_Mixin` classes and `@property` methods

We want to generate `_Mixin` classes and `@property` methods into separate macros so that `_Method` and `_Mixin` classes can be defined all together and the @property methods for each can be defined after the class definitions. This can be achieved by separating out the genration of `_Mixin` classes and `@property` methods so that they can be called in separate for loops.