Implement RFC 42: Const from shape-castable.

tests/test_hdl_ast.py

@@ -470,6 +470,25 @@ class ConstTestCase(FHDLTestCase):
         with self.assertRaises(TypeError):
             hash(Const(0))
 
+    def test_shape_castable(self):
+        class MockConstValue:
+            def __init__(self, value):
+                self.value = value
+
+        class MockConstShape(ShapeCastable):
+            def as_shape(self):
+                return unsigned(8)
+
+            def __call__(self, value):
+                return value
+
+            def const(self, init):
+                return MockConstValue(init)
+
+        s = Const(10, MockConstShape())
+        self.assertIsInstance(s, MockConstValue)
+        self.assertEqual(s.value, 10)
+
 
 class OperatorTestCase(FHDLTestCase):
     def test_bool(self):