Loom is having a big refresh to improve performance and tighten up the
concurrency model. This diff tracks those changes.
Included in the changes is the removal of `CausalCell` deferred checks.
This is due to it technically being undefined behavior in the C++11
memory model. To address this, the work-stealing queue is updated to
avoid needing this behavior. This is done by limiting the queue to have
one concurrent stealer.