Instead of having `FileName` and `DirName` introduce `Name` that can be `Name Dir` / `Name File`

This means when we `peel` we don't have to account for a case that is known to be impossible, as `peel`ing a `DirPath` would give you a `Name Dir` with:

``` purescript
peel
  :: forall a b s
   . Path a b s
-> Maybe (Tuple (Path a Dir s) (Name b))
```