Adds a type argument for State

[Synesso]

Adds a self type to State so that anyone extending State can define custom behaviour and have it available to them in any of places where kfsm references your state.

Prior to this change, it was not possible to complile the following:

sealed class Foo(to: () -> Set<State>) : app.cash.kfsm.State(to) {
  fun method() = this.subsequentStates.map { it.method() }
}

because subsequentStates was just a set of State, not a set of Foo.

Now, we can compile the following:

sealed class Foo(to: () -> Set<Foo>) : app.cash.kfsm.State<Foo>(to) {
  fun method() = this.subsequentStates.map { it.method() }
}

Because subsequentStates is a set of Foo

[Synesso]

Without this change, you would not be able to recursively call next here (or call it anywhere else in a callback for example)

[Synesso]

The real test is that this compiles, but there is a test that uses this method also just to prove it out fully.

[cwmyers]