RFC: Renaming int/uint

This RFC adopts Daniel Micay's suggestion to replace [RFC: int/uint portability to 16-bit CPUs](https://github.com/rust-lang/rfcs/pull/161) with a **single-purpose RFC to just rename `int`/`uint`.**

This RFC assumes a fixed-size integer type will become the type inference fallback and thus the language's "default integer type" and, in turn, heavily used in tutorials and libraries. See [RFC: Change integer fallback RFC to suggest i32 instead of int as the fallback](https://github.com/rust-lang/rfcs/pull/452).

**What changed since [Daniel's original RFC on this issue](https://github.com/rust-lang/rust/issues/9940)?**

* Interest in restoring integer type inference fallback, but to a fixed-size integer.
* Interest in the Rust Guide using mostly a fixed-size integer instead of pointer-sized integers.
* Mickaël Salaün found and listed some `int`/`uint` portability bugs.
* Gábor Lehel wrote an [RFC for checked arithmetic](https://github.com/rust-lang/rfcs/pull/146), making that an independent issue.
* Lots more discussions about integer type names. (See the RFC for links.)

While the new choice of names might be considered bikeshedding, moving away from long-held expectations on `int`/`uint` set by other programming languages is a usability issue.