Add an explicit Self: Trait clause to trait assoc items

[Nadrieril]

Today, items defined within a trait decl (methods, types, consts and even closures inside default method bodies) can refer to the currently-implemented trait using ImplExpr::SelfId. This can cause issues where we can't know what the SelfId clause refers to without doing additional trait resolution, e.g. in a case like this:

trait Thing {
    type Item;
    fn foo(i: Self::Item) {
        // in the closure item we forgot what `Self as Trait` is
        (|_: Self::Item| ())(i)
    }
}

To make these items less special, this PR gives them an extra Self: Trait clause argument, which will be fulfilled by trait resolution whenever they are refered to. Now only the trait declaration itself (and its associated types, while GATs aren't finalized) can use SelfId, everything else can only refer to numbered local clauses.

[W95Psp]

Thanks, that seems like a good idea to me.

However, this breaks how our engine deals with Self bounds.
So we need to schedule when we can fix it before merging.

[Nadrieril]

Oh, I indeed expected breakage but it seems that your CI passes, how come?

[Nadrieril]

Superceded by #1559.