Substantial rework of the text:

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.

Author: nikomatsakis