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)
Has there been an RFC or discussion on other ways to achieve this? We only have three sources of unsafety, could we mark up the `unsafe {}` with which we are intending on using?

`unsafe<RawDeref + MutStatic + UnsafeFn> { }`

Sorry it is a little off topic, I'm just not 100% across all the previous discussions and this seems like a similar solution.