Add `Async::Group` for straight forward list of children nodes without an associated task.

[ioquatix]

This introduces a more efficient implementation of Async::Barrier. It also makes me wonder if Async::LimitedBarrier is the right name.

Types of Changes

• New feature.

Contribution

• [x] I added tests for my changes.
• [x] I tested my changes locally.
• [x] I agree to the Developer's Certificate of Origin 1.1.