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.
A. It doesn't currently, but in the context of #146, if `#[overflow_checks(on)]`, I think it should.

Rationale: As far as I can tell `as` is meant to preserve meaning rather than representation, e.g. `5000i32 as f32` is equivalent to `5000f32` and not to `transmute::<i32, f32>(5000i32)`. Therefore if attempting to transport the meaning of the original value to the target type causes it to overflow, it should be caught.