They're not a large efficiency win at the moment, because we don't
do any delta compression.
At the moment, they simply served to compress data, but we will change
the archive mode to do that by default.
Since we're making a shared library, it should be usable by non-GPL
apps.
To allow more code sharing between the core and the tests, move them
to the LGPLv2+ too.
A few bits of test and other code are still GPL. See the new COPYING
file for more information.