rose.c3: add module to do mro resolution.
rose.c3: add to test suite, with more tests.
rose.config_tree: new module.
rose.config_tree: add more loader logic.
rose.config_tree: finallise logic.
rose.config_tree: more comments.
rose.config_tree: add basic self test.
rose.config_tree: more self tests.
rose.config_tree: more tests.

Test more optional configuration keys.
Test search path for locating configuration directories.