Abstract over Path more properly

We don't expect "Path Abs" everywhere anymore. The functions
have been made to be more generic. A user can still pass
absolute paths, so we don't lose any safety. However, some
function implementations may be more tricky.

src/HPath/IO.hs-boot

@@ -3,5 +3,6 @@ module HPath.IO where
 
 import HPath
 
-canonicalizePath :: Path Abs -> IO (Path Abs)
+canonicalizePath :: Path b -> IO (Path Abs)
 
+toAbs :: Path b -> IO (Path Abs)