Give schedulers and tasks useful names

[catamorphism]

tasks especially would be very helpful if they could be uniquely identified

[catamorphism]

Not critical for 0.6, de-milestoning

[catamorphism]

Nominating for milestone 5 (production-ready)

[graydon]

just a bug, removing milestone/nomination.

[bblum]

This should not be too hard, and would be super useful for writing test cases. All that's needed is an extra field in TaskOpts, and an Option<~str> inside the Task struct. One place to print the name would be when the task fails or is killed; can't think of any others at the moment.

[bblum]

sub-bug of #2931

[bblum]

does our notion of having multiple schedulers support naming them? it's not as obvious how to do that as it is how to name tasks.

[bblum]

I did this for tasks but not schedulers. Unassigning myself.

[huonw]

Tasks can now be named, and some tasks were given useful names in #10204 (the main task and the tasks for each test) and #10215 (calling the rustc task rustc), but there is still no way to name schedulers.

[alexcrichton]

This is done now. I don't believe that we're going to want to give names to schedulers any time soon. That's an inherently M:N concept so libstd shouldn't be exposing that, and that's the use case that this is optimizing for anyway (naming tasks).

[thestinger]

You can name a pthread on most operating systems. There are usually small length limits though.