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.
Not really. This was just an ancillary remark on an ancillary part of this proposal. The main part of the proposal (which concerns changes to the language to better accomodate 16-bit architectures) is unaffected.