RFC: Rust language version attribute

[Rendered view](https://github.com/DanielKeep/rfcs/blob/rust-ver-attr/text/0000-rust-ver-attr.md).

This RFC proposes adding a `#![rust_ver="..."]` attribute and `--rust-ver` compiler flag.

In short, this has a few goals:

- Giving users clearer, more accurate information when attempting to compile code that is unsupported by their current compiler.
- Giving the package ecosystem a formal way of dealing with language version dependencies.
- Smoothing the transition to a future, backward-incompatible release of the language.