Attempt to normalize FnDef signature in InferCtxt::cmp

[compiler-errors]

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::<()>}`

[rust-highfive]

r? @michaelwoerister

(rust-highfive has picked a reviewer for you, use r? to override)

[compiler-errors]

r? diagnostics

(or maybe @rust-lang/types?)

[jyn514]

Here is a real world use case where this would have been enormously helpful: https://mobile.twitter.com/joshuayn514/status/1557913836337786880

[estebank]

I'm r+ on this, but want to make sure another pair of eyes looks at this

r? rust-lang/types

[lcnr]

nits, then r=me

[compiler-errors]

Added ObligationCtxt::new_in_snapshot and addressed other nits.

@bors r=lcnr

[bors]

📌 Commit e5602cb has been approved by lcnr

It is now in the queue for this repository.