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.
Note that that issue is closed, and that int and uint are no longer the compiler driven default, as the fall back has been removed. https://github.com/rust-lang/rust/issues/ (i.e. it's already been decided that people aren't happy with this choice.)