Rather than allow addressing tasks either by working set ID or uuid,
with attendant performance issues, this moves the API for the working
set to a single struct that just serves as a 1-1 mapping of indexes to
UUIDs. It's up to the caller to use this information.
* Use a parser (rather than clap) to process the command line
* Outline some generic support for filtering, reporting, modifying, etc.
* Break argument parsing strictly from invocation, to allow independent testing
