hdl.ast: make Signal(name=) a keyword-only argument.

Almost no code would specify Signal(_, name) as a positional argument
on purpose, but forgetting parens and accidentally placing signedness
into the name position is so common that we had a test for it.

nmigen/test/test_hdl_ast.py

@@ -582,12 +582,6 @@ class SignalTestCase(FHDLTestCase):
         s2 = Signal(name="sig")
         self.assertEqual(s2.name, "sig")
 
-    def test_name_bad(self):
-        with self.assertRaises(TypeError,
-                msg="Name must be a string, not 'True'"):
-            # A common typo: forgetting to put parens around width and signedness
-            Signal(1, True)
-
     def test_reset(self):
         s1 = Signal(4, reset=0b111, reset_less=True)
         self.assertEqual(s1.reset, 0b111)