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.
this whole function is a search for a registered converter based on various keys.  If none of the generator function hits the StopIteration exception, that means the search failed, no suitable converter was found.