Added fast alternate io.BytesIO implementation and its test suite.
Removed old test suite for StringIO.
Modified truncate() to imply a seek to given argument value.
diff --git a/Lib/test/test_mimetools.py b/Lib/test/test_mimetools.py
index cf0e191..ef333f5 100644
--- a/Lib/test/test_mimetools.py
+++ b/Lib/test/test_mimetools.py
@@ -58,7 +58,7 @@
             s.add(nb)
 
     def test_message(self):
-        msg = mimetools.Message(io.StringIO(msgtext1))
+        msg = mimetools.Message(io.StringIO(str(msgtext1)))
         self.assertEqual(msg.gettype(), "text/plain")
         self.assertEqual(msg.getmaintype(), "text")
         self.assertEqual(msg.getsubtype(), "plain")