Remove the `void` dependency

[whitequark]

As discussed elsewhere the void dependency has caused issues with no_std before. It is also (in my subjective opinion) just kind of gross, there should be no need to have crates for language features.

Fortunately there is a ! type now which does the same thing.

[Kixunil]

! is experimental feature and not available in stable Rust. I want to change it as soon as it stabilizes. I'll keep this as a tracking issue for it.

[whitequark]

Oops, I somehow thought it already got stabilized.