Demo patch for DROP COLUMN

DON'T APPLY THIS PATCH!

Hi All,

This is the patch of what I've done so far. I'd value any feedback!

The only thing that I'm not checking (as far as I know) is ALTER TABLE/ADD
FOREIGN KEY and CREATE CONSTRAINT TRIGGER, as I'm still trying to figure out
the best place to make the change.

Every other command is updated.

Dependency support is commented out until I figure out how to do it...

It includes the most detailed regression test I've ever written, but no
expected results until I fix foreign keys.

Please have a look and see if there's anything I've overlooked...

Thanks,

Chris