Use partially ordered labels instead of numbers for edge precedence

[hendrikvanantwerpen]

Using a partially ordered set of labels for precedence is more general, and conveys more semantic information, than the current approach of using numbers. It is also more flexible when different graphs or rule sets are combined.

[CleanCut]

The github/stack-graphs repository is no longer being maintained. As per #502, I'm closing all issues and pull requests before archiving the repository.