Refactor the Engineerable code (how units create other units)

[46bit]

The codebase for this is pretty embarrassingly bad. Much of it is bad code duplicated several times. This code is disgraceful and it's beneath the standards of the Grand Budapest

[46bit]

The experimental operate-on-interfaces-not-classes branch renamed Engineerable to Engineer, which makes clearer what it is for. That's a starting point.