RFC: Scoped attributes for checked arithmetic

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[Pretty]: https://github.com/glaebhoerl/rfcs/blob/scoped-attributes-for-checked-arithmetic/active/0000-scoped-attributes-for-checked-arithmetic.md
Yes, I think that's right. Or in any case, if it precludes `undef` that's a good thing. :)

Presumably this is clear to you, but for anyone else reading: 

 * The idea is that this is very rarely a program you want to write. If you do want to write it, you should explicitly use `.wrapping_add(1)` instead of `+ 1`. And if you write it accidentally, turning on checks will catch it.

 * If the compiler saw `100i8 + 100`, it *would* be free to represent it as `fail!("overflow")`, issue a warning, or possibly even an error.

 * In C's case, the program would be allowed to do anything, from nothing, to printing "LOLWTF", to `cat /dev/urandom > /dev/sda`, to initialing the nuclear launch sequence.