Associated types and supertraits

[nrc]

These basically don't work well together. The following works:

pub trait Bar {
    type B;
}

pub trait Foo: Bar<A=int> {
    type A;
}

The following gives an error (the wrong error too), but should compile:

pub trait Bar {
    type B;
}

pub trait Foo: Bar {
    type A;
}

In this case, B should be treated like an associated type on Foo.

I am not sure if type B = int or even type B: Baz should be allowed in Foo, but I assume not (at least for now).

When the above works, we should require super trait associated types are given in type bindings, e.g., Foo<A=int, B=int>.

We should also allow Self::B in the body of Foo and if type T has bound Foo, then we should allow T::B.