mktables: Generate tables for chars that aren't in final fold pos

mktables: Generate tables for chars that aren't in final fold pos

This starts with the existing table that mktables generates that lists
all the characters in Unicode that occur in multi-character folds, and
aren't in the final positions of any such fold.

It generates data structures with this information to make it quickly
available to code that wants to use it.  Future commits will use these
tables.