Refactor Units to reduce complexity + better support conversion automation

[robfitzgerald]

The current solution for units has some capabilities for supporting high-performance conversions and type safety. However, a few aspects of the current solution create friction around automated conversion:

1. the core capability of unit types, "convert", does not come from a trait, so it cannot be abstracted over
2. state variables are accessed through the StateModel as Distance, Time, etc newtypes, which was intended to add type safety, but instead makes it challenging to abstract over, and duplicates the choice of having strongly typed unit types like DistanceUnit, TimeUnit, etc
3. unit types are disjoint, so it is complicated to abstract over all unit types

[robfitzgerald]

sketch illustrating this refactor in this gist