tower: yeah lets just redo the API one last time. this will only hurt a little

- Consistent way to compose channel handlers
- Unification of periodic events and channel events
- Code generation is simple enough
tower-freertos: support changes to task evt loop AST

* sensible enough.
tower-aadl: builds with changes to task evt AST

* Still need to overall address the issue of describing task
  event handlers, but I'm still waiting on Mike to tell me
  more about that
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