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.
Sure, but for this purpose it's just a spammy statement. We all know about
those "things" and the term IoT doesn't need to be thrown in every piece of
text on the internet. I definitely wouldn't use Rust if it claimed itself
"The language of the IoT".