ADT Lib: Persistence

[Michael van Acken <mvacken@t-online.de>]

I have uploaded my current files to
http://home.t-online.de/home/mvacken/

The `Storage' module is almost complete, meaning you can use it to
store descendants of `AcObject.Object' to file, and load them again.
It now features type name compression.  I also added a "compact" file
format: this format does not permit alien objects, but is smaller and
faster.

Still missing: resetting of `Object.mark' fields after a `Disconnect'.
Now an object can only be stored in a single session, attaching a
second writer and saving the same object again will fail.

I intend to solve this problem by removing the field `Object.mark'
altogether, and replace it in the externalization algorithm with a
separate lookup table.

-- mva