remove 'obj' system

[graydon]

Most uses of the obj system should, at this point, be replaceable with uses of the iface/impl system, with some degree of improved performance to boot. Try this in our own code (libstd and rustc) and see how far it can get. If complete, remove 'obj' support from the compiler.

[marijnh]

Assing this to myself. Deleting code is my biggest joy in life.