- Change how loom concurrency testing is triggered. To get rid of `loom` in the dependency tree
  `oneshot` pulls in, it has in addition to being gated behind `cfg(oneshot_loom)` also been made
  an optional dependency. This makes this library way smaller for downstream consumers.
  This has the downside that the crate now exposes a `loom` feature.
  DOWNSTREAM USERS ARE NOT SUPPOSED TO EVER ENABLE THIS. No stability or semver
  guarantees exist around the `loom` feature.
  This change ultimately makes no difference for any user of `oneshot` in regular usage.