Extract common work packet to schedule_common()

[qinsoon]

This PR extracts common work packets generation from each plan to a schedule_common() function. Each plan will call this. This closes #368.

[wks]

Looks good for now.

But I think we can further improve readability by using associated types.

[wks]

This function has too many type parameters. May be simplified using traits and associated types.

[wks]

The type arguments here is especially complicated, with type parameters inside type parameters.

[wks]

Here is my attempt to group together several type parameters using struct and impl. In the example below, the WorkContext trait allows the work function to take only one type parameter instead of three.

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=6474d424e8c5430e599bc58987c166c3

However, it currently crashes the Rust compiler, probably because the constant generic parameter feature is unstable. Comment out the last two lines in main will make it work.

fn main() {
    work::<StringToScreen>();
//    work::<IntToScreenDec>();
//    work::<IntToScreenHex>();
}

[qinsoon]

Here is my attempt to group together several type parameters using struct and impl. In the example below, the WorkContext trait allows the work function to take only one type parameter instead of three.

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=6474d424e8c5430e599bc58987c166c3

However, it currently crashes the Rust compiler, probably because the constant generic parameter feature is unstable. Comment out the last two lines in main will make it work.

fn main() {
    work::<StringToScreen>();
//    work::<IntToScreenDec>();
//    work::<IntToScreenHex>();
}

I worked on this a bit. We can introduce a new trait:

pub trait GCWorkContext<VM: VMBinding> {
    type PlanType: Plan<VM = VM>;
    type CopyContextType: CopyContext<VM = VM> + GCWorkerLocal;
    type ProcessEdgesWorkType: ProcessEdgesWork<VM = VM>;
}

We can use this for schedule_common(), and it works fine. I feel this type could be useful for other places as well, but I don't have a clear idea where they can be used. I would suggest doing this change in another PR. @wks