Auto merge of #37356 - cristicbz:wrapsum, r=alexcrichton

Auto merge of #37356 - cristicbz:wrapsum, r=alexcrichton

Add impls for `&Wrapping`. Also `Sum`, `Product` impls for both `Wrapping` and `&Wrapping`.

There are two changes here (split into two commits):
- Ops for references to `&Wrapping`  (`Add`, `Sub`, `Mul` etc.) similar to the way they are implemented for primitives.
- Impls for `iter::{Sum,Product}` for `Wrapping`.

As far as I know `impl` stability attributes don't really matter so I didn't bother breaking up the macro for two different kinds of stability. Happy to change if it does matter.