Vtable lookup needs more specific `Binder`

[avanhatt]

In investigating #220, I realized we are not using a specific enough Binder in our vtable instance lookup:

let instance = Instance::resolve(
            self.tcx,
            ty::ParamEnv::reveal_all(),
            def_id,
            trait_ref_t.skip_binder().substs,
        )
        .unwrap()
        .unwrap();

Rather than skip_binder(), use the more complicated rustc binder lookup.

This gets us further into compiling #220 but doesn't fix everything; splitting off to its own issue.