Change uses of Case() to Default() in preparation for RFC 39.

amaranth/lib/coding.py

@@ -41,7 +41,7 @@ class Encoder(Elaboratable):
             for j in range(self.width):
                 with m.Case(1 << j):
                     m.d.comb += self.o.eq(j)
-            with m.Case():
+            with m.Default():
                 m.d.comb += self.n.eq(1)
         return m