The only difference between all the `Message` classes is just
their `descriptor` instance variable.
So rather than create an entirely new class from scratch every time
we can simply inherit from an abstract class.
This shink each `Message` class from `1456` bytes to `944` bytes, and
the singleton class of each from `960` to `792`, for a total of
`680` bytes saved per message class, so a ~28% reduction.