RFC: int/uint portability to 16-bit CPUs

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),
explains what's meant by "default" integer types, makes 2 specific
proposals, and proposes coding style for integer sizing.
Some Atmel AVR controllers http://en.wikipedia.org/wiki/Atmel_AVR and some PIC controllers http://en.wikipedia.org/wiki/PIC_microcontroller have 16-bit address spaces. These tend to have Harvard architectures, that is, separate instruction and data memory/addresses.