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.
This suggestion makes a lot of sense in a context where overflow/underflow silently wraps around. However, if something like RFC PR #146 were to be implemented, then it would once again make sense to use types which more accurately express the range of legal values (i.e., which are self-documenting), because compiler-added checks can be enabled to catch errors where the value would go out of range. Accurate types with compiler-added assertions beats inaccurate types with programmer-added assertions.