Suggest trait implementations for references/boxes

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.

No source file, test, or entry point is named. Start by reviewing the existing reference implementations for Add, AddAssign, and Sum, then compare the possible Hasher and Debug cases mentioned in the issue. Done would require a decided, maintainable rule for which reference, mutable-reference, and Box implementations Clippy should suggest.