shared_mutex with unique lock priority
Having a shared lock where unique locks take priority would be beneficial.
std::shared_mutex gives shared locks priority when a shared lock is already held.

With a new lock with unique priority it should have the properties:

1. When a shared lock is already held and then a unique lock is attempting to be made
  - Subsequent `try_lock_shared` should immediately return false even though unique_lock isn't held yet
  - Subsequent `lock_shared` should block until unique lock is granted and relinquished
  - If a subsequent unique lock is pending while a unique lock is already held then a shared lock shouldn't be able to win the contention.

Might be worth using `WFE/WFET` for faster userspace checking of lock if it is expected to not be contended for too long.