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.
The fact that you didn't notice this typo (the lack of `raise `) means there's probably not test sufficient coverage of this happening.