Massive changes.

Separate the Conversion class into a base and a subclass; the subclass
is pretty minimal but the separation is useful for....

NewConversion:  New class that implements a somewhat different
		approach to the conversion.  This uses a table of
		instances (rather than tuples) that have more
		information than the tuples used for the older
		conversion procedure.  This allows a lot more control
		over the conversion, and it seems to be pretty
		stable.

TableEntry,
Parameter:	New classes that are used to build the conversion
		specification used by NewConversion.

TableParser:	xmllib.XMLParser subclass that builds a conversion
		specification from an XML document.

load_table():	Convenience function that loads a table from a file.

main():  Added flags --new and --old; these select which conversion is
	 used.  The default is --new.

Several fixes have been made in the old conversion as well; these were
done before writing & switching to the new conversion, and should be
archived.

The next checkin of this file will discard the old conversion; is is
kept in this checkin to allow it to be retrieved if needed, and to
avoid lossing the bugfixes that have been made to it in the interim.