Remove ProcessEdgesBase in LXR

[wks]

DRAFT:

• Give LXRStopTheWorldProcessEdges a better name, without "process edge".
• Refactor LXRStopTheWorldProcessEdges, inline process_slots and flush into do_work if possible because we don't have to follow the structure of the legacy ProcessEdgesWork trait. Move fields to local variables if possible. Replace enqueue with a local queue and a closure.

The ProcessEdgesBase was introduced to make the legacy code work. We remove it.

[wks]

LXRStopTheWorldProcessEdges is essentially the same as plan::tracing::closure::ProcessSlots (more precisely, edge-ordered slot enqueuing tracing), with the Trace being the implementation of mature_evac_trace_object and full_gc_trace_object. It also has some special handling of root edges and remembered set recorded edges.

I think it would be better to refactor LXRStopTheWorldProcessEdges after we refactor plan::tracing::closure::ProcessSlots to remove double enqueuing. It may provide more insight.