[discussion][do not merge] actors + channels architecture

[fitzgen]

This gives a sketch of what I was imagining last night.

See src/fusion.rs and src/{mag,accel,gyro}.rs for details. See SensorInput in src/fusion.rs for how multiplexing across channels with strong typing works.

The use of traits to enable mocking/testing can be pretty much reused even if we don't want to go full threading with actor + channels and instead use polling on a single thread. The one trick is that things will need to be inverted a bit so that data is pushed through, rather than pulled through.

Thoughts @jameysharp @slnovak @jeenalee @malisas ?