hdl.ir: typecheck convert(ports=) more carefully.

The `ports` argument to the `convert` functions is a frequent hotspot of
beginner issues. Check to make sure it is either a list or a tuple, and
give an appropriately helpful error message if not.

Fixes #362.

nmigen/test/test_hdl_ir.py

@@ -304,6 +304,16 @@ class FragmentPortsTestCase(FHDLTestCase):
                 msg="Only signals may be added as ports, not (const 1'd1)"):
             f.prepare(ports=(Const(1),))
 
+    def test_port_not_iterable(self):
+        f = Fragment()
+        with self.assertRaises(TypeError,
+                msg="`ports` must be either a list or a tuple, not 1"):
+            f.prepare(ports=1)
+        with self.assertRaises(TypeError,
+                msg="`ports` must be either a list or a tuple, not (const 1'd1)" +
+                " (did you mean `ports=(<signal>,)`, rather than `ports=<signal>`?)"):
+            f.prepare(ports=Const(1))
+
 class FragmentDomainsTestCase(FHDLTestCase):
     def test_iter_signals(self):
         cd1 = ClockDomain()