New patch for 3.4 that uses a private attribute on CodecInfo and a private class method to set it appropriately (as I believe that is a better approach than changing the signature of CodecInfo.__init__ at this point, especially if we end up pursuing the codec type map idea in 3.5)

This version also updates the tests to check for the appropriate error messages.

The integration into the text model related methods is that same as in the proof of concept: a parallel private text-encoding-only C API that is used in preference to the general purpose codec machinery where appropriate.

If there aren't any objections to this approach, I'll commit this one tomorrow.