create like Mutex.create, but creates an error-checking mutex.
Locking a mutex twice from the same thread, unlocking an unlocked mutex,
or unlocking a mutex not held by the thread will result in a Sys_error
exception.
val critical_section : t ‑> f:(unit ‑> 'a) ‑> 'aval update_signal : t ‑> Condition.t ‑> f:(unit ‑> 'a) ‑> 'aval update_broadcast : t ‑> Condition.t ‑> f:(unit ‑> 'a) ‑> 'aval try_lock : t ‑> [> `Acquired | `Already_held_by_me_or_other ]