Both Issue #14758 and Issue #9940 call for RFCs.

This RFC summarizes those discussions, explains the core issue of
code portability to 16-bit CPUs (also of 64-bit code to 32-bit CPUs),
explain what's meant by "default" integer types, makes 2 specific
proposals, and proposes usage style for integer sizing.

* Crisper/broader motivation.
* "The smallest integers that span the address space" is clearer than
"pointer-sized integers".
* More concise.
* More "not in scope" items.

* Recommended unsigned or signed integer types for numbers that
should not be negative -- depending on whether Rust provides integer
overflow checking.

* Crisper integer style guideline section.