Don't expose GChecksum in APIs. Add a new stream class which allows
us to pass an input stream somewhere, but gather a checksum as it's
read.
Move some bits of the internals towards binary csums.
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.