lib.wiring: make values of In and Out be strings "In" and "Out".

Their `str()` and `repr()` values are already that; and the 0 and 1
don't make sense. The RFC leaves it unspecified.

amaranth/lib/wiring.py

@@ -15,8 +15,8 @@ __all__ = ["In", "Out", "Signature", "PureInterface", "connect", "flipped", "Com
 
 
 class Flow(enum.Enum):
-    Out = 0
-    In = 1
+    Out = "Out"
+    In = "In"
 
     def flip(self):
         if self == Out:

tests/test_lib_wiring.py

@@ -26,6 +26,10 @@ class FlowTestCase(unittest.TestCase):
         self.assertEqual(str(Flow.In), "In")
         self.assertEqual(str(Flow.Out), "Out")
 
+    def test_flow_value(self):
+        self.assertEqual(Flow.In.value, "In")
+        self.assertEqual(Flow.Out.value, "Out")
+
 
 class MemberTestCase(unittest.TestCase):
     def test_port_member(self):