Replace `Void` in `sys` with never type This PR replaces several occurrences in `sys` of the type `enum Void {}` with the Rust never type (`!`). The name `Void` is unfortunate because in other languages (C etc.) it refers to a unit type, not an uninhabited type. Note that the previous stabilization of the never type was reverted, however all uses here are implementation details and not publicly visible.