Idea: Keep count / identifiers of all Ref/Deferred in runtime

This probably goes into CE3 territory, but it would be nice if we had more productionreadiness-friendly utilities in the runtime. It might be a bit too heavy in performance-sensitive code, but for most users having an extra reference for each Ref or Deferred would probably not be that much memory pressure.

Some extra ideas in the area:

• being able to access the identity of the fiber that spawned/changed a Ref/Deferred, the fibers waiting on a Deferred
• looking at the tree of fibers in the runtime, along with identities of fibers waiting on them / having canceled them

On top of such registries, we could probably build some useful debugging/monitoring tools, an example of something similar in the wild is https://github.com/ScalaConsultants/panopticon-tui.

The proposal concerns runtime registries for Ref and Deferred objects, fiber identities, waiting fibers, and fiber relationships. No files, tests, or entry points are named; first identify the relevant runtime subsystem and define the scope and performance requirements before determining what completion would mean.