Re: Transparent column encryption

Updated patch, to resolve some merge conflicts.

Also, I added some CREATE DDL commands. These aren't fully robust yet,
but they do the basic job, so it makes the test cases easier to write
and read, and they can be referred to in the documentation. (Note that
the corresponding DROP aren't there yet.) I also expanded the
documentation in the DDL chapter to give a complete recipe of how to set
it up and use it.