RFC: Allow safe access to some `static mut` vars

[Rendered](https://github.com/alexcrichton/rfcs/blob/safe-static-mut/active/0000-safe-static-mut.md)
FWIW, once https://github.com/rust-lang/rust/issues/15701 is implemented it could be a lint, interpreting things like `#[behaviour(raw_deref, mut_static)] unsafe { ... }`.