Process stack needs to store data

[vertexclique]

We want to carry actual data with process stack but we can't because of size bounds are varying. We need to find a way to do this to make simpler callback implementation.

pub struct ProcStack<S> {
    pub state: S,

    pub before_start: Option<Arc<dyn Fn(&mut S) + Send + Sync>>,
    // ...
}

[vertexclique]

@azastrael mentioned to not move whole environment to Fn. Creates unnecessary copies.