Fixed Gc::clone, implemented Gc::ptr_eq, fixed test-building warnings
auto merge of #11369 : kvark/rust/master, r=alexcrichton

-Fixed Gc::clone to not require a Clone implementation for T
-Implemented Gc::ptr_eq to match Rc::ptr_eq, added a test for that
-Marked some stuff with #cfg(not(test)) that was reporting warnings when building tests