Stashes a normalization callback in InferCtxt so that the signature we get from tcx.fn_sig(..).subst(..) in InferCtxt::cmp can be properly normalized, since we cannot expect for it to have normalized types since it comes straight from astconv.

This is kind of a hack, but I will say that @jyn514 found the fact that we present unnormalized types to be very confusing in real life code, and I agree with that feeling. Though altogether I am still a bit unsure about whether this PR is worth the effort, so I'm open to alternatives and/or just closing it outright.

On the other hand, this isn't a ridiculously heavy implementation anyways -- it's less than a hundred lines of changes, and half of that is just miscellaneous cleanup.

This is stacked onto #100471 which is basically unrelated, and it can be rebased off of that when that lands or if needed.

The code:

trait Foo { type Bar; }

impl<T> Foo for T {
    type Bar = i32;
}

fn foo<T>(_: <T as Foo>::Bar) {}

fn needs_i32_ref_fn(f: fn(&'static i32)) {}

fn main() {
    needs_i32_ref_fn(foo::<()>);
}

Before:

   = note: expected fn pointer `fn(&'static i32)`
                 found fn item `fn(<() as Foo>::Bar) {foo::<()>}`

After:

   = note: expected fn pointer `fn(&'static i32)`
                 found fn item `fn(i32) {foo::<()>}`