Status tracker for coffea.compute

[nsmith-]

The initial implementation is described in #1470 along with some skeleton code for the various components. To better organize the discussion here, I'm breaking out some TODOs into separate issues to help finish the coffea.compute design. We can implement them by making PRs to the compute branch.

• Sketch a prepare -> skim -> process workflow to understand Computable-chaining
• Define NanoEvents map_steps interface (how does dask-awkward option look?)
• Define protocol for receiving a list of failed tasks with exceptions (promote FailedTaskElement to protocol)