Multiple improvements to tab completion. I refactored the API quite a bit, to retain readline compatibility but make it more independent of readline. There's still more to do in cleaning up our init_readline() method, but now the completer objects have separate rlcomplete() and complete() methods. The former uses the quirky readline API with a state flag, while the latter is stateless, takes only text information, and is more suitable for GUIs and other frontends to call programatically. Made other minor fixes to ensure the test suite passes in full. While all this code is a bit messy, we're getting in the direction of the APIs we need in the long run.