[plt-scheme] contract library reoganization

I've re-organized the contract library so that it no longer exports
class & object-related contracts, instead those are exported by (lib
"class.ss").

This is currently breaking a few planet packages, so if people need
more time to fix this, I can switch it back temporarily. (The fix
should be simple: just add (require (lib "class.ss")) to the file in
question and upload a new planet package).

Please let me know.

Robby