Refactoring-aware incrementality

[flupe]

Because the build graph is fully static, we could build some kind of Merkle tree
before running the generator. If all the build tasks append their hash to their local cache (by convention), we should be able to detect whether a task (or its children) has changed since the last run, and thus possibly force recompilation.

May or may not happen before the next version, but definitely for the one after that.