Remove typed-paths interface and use FilePath for everything

[patrickt]

Though FilePath is very flawed, especially on Windows, typed paths were a failed experiment. They did not actually catch any bugs; the best thing they did was expose some very weird behavior in certain functions, ones that we were unable to change (without huge renovations) lest command-line invocations break. There is a new FilePath type coming, one based on ShortByteString and officially blessed by a GHC working group, and we should just switch back to FilePath so that we can transition to that when possible.