As a part of the cleanup of MessageFormat, here's what we have planned.

1. Have two interfaces that UFormat inherits from: FormatOnly and ParseOnly (if anyone has better names, let me know!)
2. Replace API parameter types with these where we can (where it is an acceptable broadening)
3. Create a MessageFormatOnly that only does formatting, thus only implements FormatOnly
   
1. It only does placeholder substitution for strings, and parse of strings.
   
2. It has a registry of FormatOnlys that it uses to format based on placeholder-keys / types, but it is unpopulated.
3. MessageFormat inherits from it, and populates the registry.

Note: longer term, I think we might want to have a more generalized interface: an object that can match at a given point in a string, and (logically) return back a set of results and how far through the string each represented, plus flagged partial-unique results.

That is, with the contents {Jan => M1, January => M1, June => M6, July => M7}, we'd have the following results (pos, string => output-matches)