A unique reference.
A value of `'a t` won't be created twice (but can be shared/copied), which makes it ideal for coordination between processes.
Normally, you'd use a `'a Ref.t` to identify outgoing/incoming message pairs, but they can also be used for type-equalities. If two refs of type `'a Ref.t` and `'b Ref.t` are equal, then you can use `Ref.type_equal` to obtain a type-level witness that proves that `'a` and `'b` are equal.
val make : unit -> 'a t`make ()` creates a new unique ref. The type of the ref may be inferred from context or explicitly set.
val pp : Stdlib.Format.formatter -> 'a t -> unit`equal ref1 ref2` returns true if both references are the same, regardless of the type they hold.
If the types are different, you may want to use `type_equal` instead to prove type equality.