Support a canonical() method, implementable by a derived class, to be

Support a canonical() method, implementable by a derived class, to be
applied to all filenames before they are compared, looked up in the
breaks dictionary, etc.  The default implementation does nothing --
it's implented as fast as possible via str().  A useful implementation
would make everything a absolute, e.g. return os.path.normcase(
os.path.abspath(filename)).