Remove ProcessEdgesBase::worker

[wks]

The ProcessEdgesBase::worker field is a *mut GCWorker<VM>. This is unsafe.

pub struct ProcessEdgesBase<VM: VMBinding> {
    pub edges: Vec<Address>,
    pub nodes: Vec<ObjectReference>,
    mmtk: &'static MMTK<VM>,
    worker: *mut GCWorker<VM>,  // This is unsafe. Remove it
    pub roots: bool,
} 

ProcessEdgesWork needs to access the worker instance because

1. XxxxxxSpace::trace_object needs a GCWorker<VM> reference to get the CopyContext, and
2. flush() needs a GCWorker<VM> to submit or execute ScanObjects work packets.

Currently, ProcessEdgesWork attempts to give itself access to GCWorker by holding a *mut GCWorker<VM> in the ProcessEdgesBase::worker field. This is unsafe, because the work packet is not associated to a GCWorker until it is executed by a GCWorker. The access to the *mut GCWorker<VM> pointer is invalid before do_work starts, and after do_work finishes.

In idiomatic Rust, if the GCWorker is only valid to access during the execution of gc_work, it should be passed to gc_work as a &mut GCWorker<VM> reference. Rust's borrowing semantics will ensure that it is only borrowed during the execution of gc_work.

Actually GCWork::do_work already has a worker: &mut GCWorker<VM> parameter. So we can pass it through levels of function calls to give them access to GCWorker.

This PR attempts to do this.

DRAFT: I added &mut GCWorker parameter to too many functions, but only a few functions actually use it. This may indicate that there is still some room for refactoring. One possibility is to introduce a struct that has a lifetime parameter 'w and contains a &'w mut GCWorker, and use that struct as the self, but only during the execution of do_work.