mktables: Revamp the compare versions functionality

mktables: Revamp the compare versions functionality

This functionality is rarely used, but enables someone to see what
Unicode has changed between releases X and Y, without the clutter of the
things that are added after X came out.  In other words it compiles
release X using Y's rules.  To use it, you must go in and edit mktables
to specify to use this; so it is intended only for a developer who wants
to look at Unicode history.  One use I've done is to look at the beta
version of a new release to compare with the previous official one.
This allows me to find typos, and unintentional changes and report them
back to Unicode.

This commit significantly overhauls this feature, giving better results
than before.