v2.0.1: feat: use generic classes to interpret the promise types

The previous version required that promise values were nilable.
i.e. Promise.new(String) == Promise::DeferredPromise(String?)

This is no longer the case
Promise.new(String) == Promise::DeferredPromise(String)

This was achieved by:

1. Using generic classes to isolate the return value types of promise callbacks
2. Using macros to differentiate between promise and regular return values without creating a union type
3. Detecting NoReturn where a callback always raises and creating a Promise::DeferredPromise(Nil) for this case

Otherwise the library functions exactly as before.