Implement using eventbus

[hannahhoward]

Goals

Demonstrate that the fsm can be built using event bus

Prove the interface by doing so

Implementation

• Use our test event bus interface and demonstrate that the whole go-statemachine can be built with the event bus.
• Introduce a "core" package that works at the eventbus level
• Implement the base statemachine with this package
• Right now FSM continues to sit on top of statemachine
• However, we can refactor it to sit on top of core, at which point we can introduce a lot of other cool features (like using other external statemachine's events in transitions)