refactor converters

In preparation for adding new definition and value sources (argparse, python modules, ...) I've refactored the converters.  They're now sorted into to types: to string and from string.  Both types now have dispatch functions that will relieve clients of this module from having to find the appropriate converter manually.
I don't like how we here depend on a private function in an imported library. Can you rename `_arbitrary_object_to_string` to `arbitrary_object_to_string` in converters.py?