Fat interface files?

Hi Edward, While chatting with the ghc-ide folks recently I realized that it would be useful to be able to preserve Core such that compilation can be restarted (e.g. to be pushed down the bytecode pipeline to evaluate TH splices). As I recall this is precisely what you implemented in your "fat interface file" work. Do you recall what the state of this work was? Do you have a branch with last-known-good work? Do you recall any tricky questions that remained outstanding? Cheers, - Ben