Implement stable caching (and tests) for colocation compilation.

[rwjblue]

The main implementation in #249 did the semi-naive thing of building the output tree every time without caching. Before we can enable this by default for all scenarios, we must implement a caching system that ensures the output tree is stable if none of the things changed here are modified.

[rwjblue]

Done in #384

[btecu]

@rwjblue Was this actually implemented in #380?

[rwjblue]

Do'h 🤦‍♂, yes thank you @btecu