mandate checking in debug mode.

and that panics may occur only on some branches.

1. Clarify the aims of the RFC and generally streamline the text.
2. In the event that no panic occurs, define the results of overflow.
3. Be less generous about the compiler's ability to defer panics.
4. Move the wrapping operations out of the prelude and into `std::num`.
5. Elaborate on the objections raised to the previous draft
   and respond to them.

that not having checks enabled is not the same as having them explicitly
*disabled*.

of division and shift are out of scope for this RFC.