Fix #80: Convert contribs to use ASDF to load
As it says, convert all the contribs to use ASDF to load them. Many contribs already had an asd file so we basically just had to rename foo.asd to contrib-foo.asd to keep backward compatibility with the old contrib names. (And update the defsystem name to match.)
Added an asd file for packed-sse2 and unix.