make Console and startUi more generic

[louispan]

1. in startUi, need to generalize the TVar D.Decimal (specific to passing threshold values back to the signal network).
2. I'd like to try to implement another version of the console app using the Brick framework, and seeing how much code can be reused.

[louispan]

Regarding 1., this currently accomplished by modeling both (1. sensor data from external sources) and (2. state changes from gadget actions) as Producer a (StateT s STM) () which means the state effects can be impulsively merged together.