FcCharSetSerialize was using wrong offset for leaves. Make fc-cat work.

FcCharSetSerialize was using wrong offset for leaves. Make fc-cat work.

FcCharSetSerialize was computing the offset to the unserialized leaf,
which left it pointing at random data when the cache was reloaded.

fc-cat has been updated to work with the new cache structure.

Various debug messages extended to help diagnose serialization errors.