Fix UnselectedProjectionTy parameters

[scalexm]

There were two problems with how UnselectedProjectionTy parameters where handled:

Firstly, they were in the wrong order with respect to those of ProjectionTy, i.e. the bound parameters of the associated type value should come first, then the Self type should come. For example, given:

trait Foo {
    type Item<'a>;
}

impl<T> Foo for T {
    type Item<'a> = Ref<'a, T>;
}

we would issue clauses like UnselectedNormalize('a::Item<T> -> Ref<'a, T>) :- ... instead of UnselectedNormalize(T::Item<'a> -> Ref<'a, T>) :- ....

Secondly, if the trait ref had any more parameters (in addition of the Self parameter), we would add those extra parameters to the UnselectedNormalize clause in ir::AssociatedTyValue::to_program_clauses. For example, given:

trait Foo<U> {
    type Item;
}

impl<T, U> Foo<U> for T {
    type Item = T;
}

we would issue clauses like UnselectedNormalize(T::Item<U> -> T) :- ... instead of UnselectedNormalize(T::Item -> T) :- ....

This would eventually give zipping things of mixed type panics. Fixes #70.

[nikomatsakis]

Looks good. Do you have a test case for #70?

[nikomatsakis]

Ah, I see, self comes first because it is of the trait.

[scalexm]

Added two tests that would fail with current master branch (@nikomatsakis).