pyloxi: slightly generalize alignment

The _pack and _unpack templates should not be matching on particular class
names. Move the decision up a few levels to codegen.
diff --git a/py_gen/templates/generic_util.py b/py_gen/templates/generic_util.py
index eca828c..28d3b5f 100644
--- a/py_gen/templates/generic_util.py
+++ b/py_gen/templates/generic_util.py
@@ -109,6 +109,12 @@
             raise loxi.ProtocolError("Buffer too short")
         self.offset += length
 
+    def skip_align(self):
+        new_offset = (self.offset + 7) / 8 * 8
+        if new_offset > len(self.buf):
+            raise loxi.ProtocolError("Buffer too short")
+        self.offset = new_offset
+
     def is_empty(self):
         return self.offset == len(self.buf)