Add support for shared state context

[Pyxus]

Added

• Added FrayState._enter_tree_impl() method. This method will be invoked when the state is added to the hierarchy.
• Added read-only context which is shared to all states. The context can be provided when initializing a state machine.

  state_machine.initialize({
   key1 = value1,
   key2 = value2
  }, FrayCompoundState.builder()
   .add_state(...)
   .transition(...)
  )

Changed

• FrayStateMachine is no longer initialized by setting the root. Instead the root should be passed to the initialize method. The initialize method also contains a parameter to pass context.

  state_machine.initialize(context, root)

• FrayState._ready_impl() is no longer called when the state is added to the hierarchy. It is now called when the state is initialized

Fixed

• FrayStateMachine now invokes FrayState._input() and FrayState._unhandled_input(). In the recent state management refactor I added input added these virtual functions but some how forgot to actually invoke them.

Related Issues

Fixes #72