the problems with the original implementation was mostly around error handling. sources had to be tracked manually and provided to each incremental output. the `hasArtifact` check was frequently forgotten. this has been re-abstracted through `incr.work()`, which is given an `io` object. all fs reads and module loads go through this interface, which allows the sources to be properly tracked, even if it throws.
closes#12
chloe caruso
25f01d878c