Can't name parallel Tasks

None of the overloads for `Task.par(...)` allow specifying a string description. The underlying implementation, `Par2Task`, etc. have the `desc` field, which is simply set to something like `par`, `par2`, etc. When looking at a trace, this makes it hard to understand where a parallel task is defined in the code.

Is there a fundamental reason why there are no overloads for `Task.par(...)` which accept a human-readable description? I'm happy to add these overloads if there's no objection.

Start by reading the Task.par(...) overloads and the Par2Task and related implementations mentioned in the issue, focusing on how their desc fields are assigned. The work is complete when parallel-task overloads accept human-readable descriptions and those descriptions appear in traces.

Written by the indexing model from the issue text.