Suggest trait implementations for references/boxes

[clarfonthey]

Sort of an awkward way to word this, but it'll make a bit more sense with an explanation.

Right now, there are implementations for &T + T, T + &T, and T += &T for integer types. There should be some nice list of traits where there should be suggestions to add implementations for references as well.

Just off the top of my head:

• Add, etc. should have Add<&U> for T and Add<U> for &T if Add<U> for T is available.
• AddAssign, etc. should have AddAssign<&U> for T if AddAssign<U> for T is available.
• Sum, etc. should have Sum for &T if Sum for T is available.

There may be a way to detect a list of traits where an impl for T should suggest an impl for &T, &mut T, and Box<T> as well, like Hasher and Debug. I'm not sure how we could suggest which traits work for this, but I figured I'd add it to the discussion.

These "convenience" impls are nice to have and there are a lot of cases where these are added to the standard library when they are forgotten. It'd be nice if we could suggest them via clippy.

[ada4a]

@rustbot label +A-lint