hdl.rec: make Record inherit from UserValue.

Closes #354.

nmigen/test/test_hdl_ast.py

@@ -916,6 +916,14 @@ class UserValueTestCase(FHDLTestCase):
         self.assertEqual(uv.shape(), unsigned(1))
         self.assertEqual(uv.lower_count, 1)
 
+    def test_lower_to_user_value(self):
+        uv = MockUserValue(MockUserValue(1))
+        self.assertEqual(uv.shape(), unsigned(1))
+        self.assertIsInstance(uv.shape(), Shape)
+        uv.lowered = MockUserValue(2)
+        self.assertEqual(uv.shape(), unsigned(1))
+        self.assertEqual(uv.lower_count, 1)
+
 
 class SampleTestCase(FHDLTestCase):
     def test_const(self):

nmigen/test/test_hdl_xfrm.py

@@ -620,3 +620,12 @@ class UserValueTestCase(FHDLTestCase):
             )
         )
         """)
+
+
+class UserValueRecursiveTestCase(UserValueTestCase):
+    def setUp(self):
+        self.s = Signal()
+        self.c = Signal()
+        self.uv = MockUserValue(MockUserValue(self.s))
+
+    # inherit the test_lower method from UserValueTestCase because the checks are the same