hdl.ast: improve repr() for Shape.

The default __repr__() from typing.NamedTuple does not include
the module name, so the replacement (which uses the preferred syntax
for specifying these shapes) doesn't either.

nmigen/test/test_hdl_ast.py

@@ -39,6 +39,10 @@ class ShapeTestCase(FHDLTestCase):
                 msg="Width must be a non-negative integer, not -1"):
             Shape(-1)
 
+    def test_repr(self):
+        self.assertEqual(repr(Shape()), "unsigned(1)")
+        self.assertEqual(repr(Shape(2, True)), "signed(2)")
+
     def test_tuple(self):
         width, signed = Shape()
         self.assertEqual(width, 1)