Rollup merge of #58975 - jtdowney:iter_arith_traits_option, r=dtolnay

Rollup merge of #58975 - jtdowney:iter_arith_traits_option, r=dtolnay

Implement `iter::Sum` and `iter::Product` for `Option`

This is similar to the existing implementation for `Result`. It will take each item into the accumulator unless a `None` is returned.

I based a lot of this on #38580. From that discussion it didn't seem like this addition would be too controversial or difficult. One thing I still don't understand is picking the values for the `stable` attribute. This is my first non-documentation PR for rust so I am open to any feedback on improvements.