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
Overflow happens in Rust code and gets passed to C code which neglects to do a bounds check, for instance. Of course, anything like that should either have a bounds check added on the Rust side or be an `unsafe fn`, so this is not *truly* a distinct case from "`unsafe` blocks", but the point is that it's better to be safe than sorry.