Add `Domain.join`

[art-w]

It's a bit of a pain to adapt existing code to dscheck because it doesn't have the standard Domain.spawn type (and Domain.join is missing). It would be super neat if enabling tracing only required this prelude:

open Dscheck

(* or *)
module Atomic = Dscheck.Atomic
module Domain = Dscheck.Domain

(with Domain.join added, I suspect that final wouldn't be required anymore?)