Use associated types in binop traits

[japaric]

binop traits (Add, Sub, etc) now have an associated type for their return type. Also the RHS input parameter defaults to Self (except for the Shl and Shr traits). For example, the Add trait now looks like this:

trait Add<Rhs=Self> {
    type Result;

    fn add(self, Rhs) -> Self::Result;
}

This breaks all existing implementation of these traits.

[breaking-change]

---

Needs a snapshot that contains PR #20374

There are some cfail test that are ICEing, I have marked those with a FIXME. We can land this after a snapshot, or wait for fix of those cfail ICEs before landing this.

r? @aturon

[rust-highfive]

r? @nikomatsakis

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[aturon]

YES! Happy new year :-)

[nikomatsakis]

Nice! 💃

[aturon]

r=me once we're ready to land this.

[japaric]

Closing in favor of #20410