Add Swift macro (requires Swift v5.9)

[tinder-cfuller]

The added Swift macro provides StateMachineHashable conformance automatically for Swift version 5.9 and above.

Code Example:

@StateMachineHashable
indirect enum State {

     case locked(credit: Int), unlocked, broken(oldState: State)
}

@StateMachineHashable
enum Event {

     case insertCoin(Int), admitPerson, machineDidFail, machineRepairDidComplete
}

Older Swift versions will still require boilerplate implementation for State or Event enumerations defined with associated values – so that information remains in the README.

[tinder-danielhall]

Very very cool! Excellent work, Chris! I left a few comments for consideration 👍