Introduce a `Void` type for uncolored edges or states

[leonbohn]

We should represent uncolored edges or states with a separate type that is not just (). As the empty type () implements Color, this leads to some annoyances if we want to properly implement the "dropping" of colors during implement of the collecting of transition systems.

The rough idea of this change should follow along a type like this

pub struct Void;
impl<C: Color> From<C> for Void {
    fn from(_: C) -> Self {
        Void
    }
}