Fix Nixpkgs evaluation on Functions

Nix says that { f = x: x; } == { g = y: y; }. The rule says to ignore
functions when comparing attribute sets or lists. This commit follows
this rule. However, it is still true that:

(x: x) /= (y: y)

I have also added some tests that check for these cases.

Fixes #323

src/Nix/Value.hs

@@ -219,7 +219,10 @@ isClosureNF _ = False
 
 thunkEq :: MonadThunk (NValue m) (NThunk m) m
         => NThunk m -> NThunk m -> m Bool
-thunkEq lt rt = force lt $ \lv -> force rt $ \rv -> valueEq lv rv
+thunkEq lt rt = force lt $ \lv -> force rt $ \rv ->
+  case (lv, rv) of
+    (NVClosure _ _, NVClosure _ _) -> pure True
+    _ -> valueEq lv rv
 
 -- | Checks whether two containers are equal, using the given item equality
 --   predicate. If there are any item slots that don't match between the two

tests/EvalTests.hs

@@ -277,6 +277,15 @@ case_fixed_points_attrsets =
       in fix f
     |]
 
+case_function_equals1 =
+    constantEqualText "true" "{f = x: x;} == {f = x: x;}"
+
+case_function_equals2 =
+    constantEqualText "true" "[(x: x)] == [(x: x)]"
+
+case_function_equals3 =
+    constantEqualText "false" "(x: x) == (x: x)"
+
 -- jww (2018-05-02): This constantly changes!
 -- case_placeholder =
 --   constantEqualText