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.
I'm unclear on Issue #11831 "Decide whether to keep pointer sized integers as the default", "make a firm decision one way or the other on whether we want to commit to int and uint being pointer sized".

Q. "Default" means for the compiler not for programmers, right?

Q. When the issue was closed, was that a commitment to keeping int and uint as pointer-sized?

If so, it's not my intention to reopen a design decision, just to propose in that case making int/uint be at least 32 bits.