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.

test/HPath/IO/ToAbsSpec.hs

@@ -0,0 +1,27 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+
+module HPath.IO.ToAbsSpec where
+
+
+import Test.Hspec
+import HPath
+import HPath.IO
+
+
+
+spec :: Spec
+spec = describe "HPath.IO.toAbs" $ do
+
+    -- successes --
+    it "toAbs returns absolute paths unchanged" $ do
+      p1 <- parseAbs "/a/b/c/d"
+      to <- toAbs p1
+      p1 `shouldBe` to
+
+    it "toAbs returns even existing absolute paths unchanged" $ do
+      p1 <- parseAbs "/home"
+      to <- toAbs p1
+      p1 `shouldBe` to
+
+