RFC for removing integer inference fallback

Rust infers primitive integer types, but when it can't find a solution it currently defaults to `int`. This is often seen as a potential subtle footgun. This RFC describes how to remove the feature, per https://github.com/mozilla/rust/issues/6023.
I believe that is going to be the rule. Enums are the smallest type (probably not specced as such), unless using `#[repr(...)]`.