Apparently we do still need .ghci with cabal repl
Restrict FileName to Directory again
Make Path only a semigroupoid for now

'Edge' felt a bit like a hack and I don't need it anymore since
refactoring 'path' in the previous commit.  We can always bring back
Category later if it makes sense.
Rename drive operator
Export Path constructors
Move modules to Data namespace

System.Path is used by at least pathtype, fsutils and MissingH, the last
one which is actively maintained and currently has 80 reverse
dependencies on hackage.  The Data namespace seems OK because our Path
type is strongly typed and rich in structure.
Don't split RawFilePath unless we're decoding
Missing addTrailingPathSeparator