New language feature improvements

[JamesYFC]

When Unity supports newer language features like covariant returns, I will be able to simplify the codebase and improve quite a few aspects of this library. Though that will likely bump up the supported Unity version as well. We'll leave the previous version in another branch.

After this, I will look into new features and improvements such as...

• Supporting UniTask/task based delegates on some functions
  • (it is currently possible to provide async methods, the state machine will just run them and forget about them)
• Make passing event args nicer.
• Look into nested state machines
• FSM-wide state
• various other API improvements
  and more...

No promises that everything will be feasible but I will certainly be looking into them when it's time.