hdl.ast: add name_suffix=".." option to Signal.like().

This simplifies creation of related signals with nice names during
metaprogramming, e.g.

  def make_ff(m, sig):
      sig_ff = Signal.like(sig, name_suffix="_ff")
      m.d.sync += sig_ff.eq(sig)
      return sig_ff

nmigen/test/test_hdl_ast.py

@@ -483,6 +483,8 @@ class SignalTestCase(FHDLTestCase):
         self.assertEqual(s6.shape(), (4, False))
         s7 = [Signal.like(Signal(4))][0]
         self.assertEqual(s7.name, "$like")
+        s8 = Signal.like(s1, name_suffix="_ff")
+        self.assertEqual(s8.name, "s1_ff")
 
 
 class ClockSignalTestCase(FHDLTestCase):