RFC: Scoped attributes for checked arithmetic

[Pretty][Pretty]

[Pretty]: https://github.com/glaebhoerl/rfcs/blob/scoped-attributes-for-checked-arithmetic/active/0000-scoped-attributes-for-checked-arithmetic.md
It seems that this operator list needs to include `as` conversion to an integer type. Otherwise computing a value in one numeric type then converting would accidentally bypass the overflow check.

https://github.com/rust-lang/rfcs/pull/161#discussion_r14859403