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.
yes, it is a value to be converted.  I use the term "subject" in the manner of the "subject" of a sentence: it is the thing on which an action will happen.