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)
This has come up in the past before, although there haven't been any concrete proposals in the area, more of just ideas. For now though, the RFC is written with the assumption that this does not exist (but it would be quite nice!)