Provide distinct event number for types defined by reference

Prior to this commit, message types defined by reference were exposed
to Rust as type aliases, such as `type KeyReleaseEvent =
KeyPressEvent;`.  As a result, they shared a single implementation of
`BaseEvent`, erroneously sharing the same `BaseEvent::NUMBER`.

This commit updates the code generation to instead expose these types
as a wrapper struct, such as `struct KeyReleaseEvent(KeyPressEvent);`.
These wrapper structs expose the same methods, but have an independent
implementation of `BaseEvent`.  This allows these events to be
constructed, such as for use in `SendEvent`.

Fixes #214