Deprecate direct use of Unicode db files

Deprecate direct use of Unicode db files

Unicode::UCD has been enhanced to provide a stable API for accessing the
Unicode data base.  Some applications have needed data that has
previously only effectively been available from files stored on disk.

It may be that some time in the future we will want to change or remove
these files, so a warning has been added to their headers to that
effect.

Already, it would have been more convenient to change the
formats somewhat in some of these, and I have had to do some hoop
jumping to avoid this.

I don't see any call to change them now for many releases down the road,
but for example, we may choose to store more of the db in memory, and
would no longer need these.