Add scoped threads to Call for Testing

[m-ou-se]

No description provided.

[U007D]

Hi, @m-ou-se. Can you point me to anything outlining the kind(s) of testing you would like to see? I will include this in TWiR-443.

[m-ou-se]

Nothing specific I think. The std::thread::scope function is available in nightly, and it'd be great if folks could use/test it in whatever way they like.

It might be good to include a link the docs: https://doc.rust-lang.org/nightly/std/thread/fn.scope.html (But the tracking issue also links that. And back.)

[U007D]

Closing this PR as I have incorporated the data from this PR and from the comment thread into TWiR #443.

Going forward I would like to create a call-for-testing label that authors such as yourself can use. I will query for these each week to pick up all the relevant items.