Message1465

I think we can actually remove the self instanceof PyUnicode as well while we're working on this; PyUnicode exposes a join method of its own that always returns PyUnicode.

I only see a couple things you'd have to change to have str_join return PyString: PyString.join would have to return str_join(seq).string and PyUnicode.unicode_join would have to replace the result with an instance of PyUnicode if the str_join didn't require it.  That's probably better because I think it means we could remove the custom method implementation code for join in str.expose in addition to getting rid of a second iteration.