reduce conversion table (.cnv) file size by omitting part of the fromUnicode data

Idea: Optionally reduce the size of MBCS conversion tables by omitting part of the fromUnicode data and reconstituting it at converter load time. This works for roundtrip mappings which can be enumerated from the toUnicode data. Fallbacks would be moved to the extension table.

Trade off file size vs. load speed and heap memory usage.

We could later (separately) add a cache hook to ICU to share such data across processes.