librustc: Allow trait bounds on structures and enumerations, and check

them during kind checking.

This implements RFC #11.

Closes #15759.

src/test/compile-fail/object-does-not-impl-trait.rs

@@ -15,4 +15,5 @@
 trait Foo {}
 fn take_foo<F:Foo>(f: F) {}
 fn take_object(f: Box<Foo>) { take_foo(f); } //~ ERROR failed to find an implementation of trait
+//~^ ERROR failed to find an implementation
 fn main() {}