Format: replace bespoke queue for Stdlib.Queue
Format: replace bespoke queue for Stdlib.Queue

This reduces the volume of code in Format and adds clarity by using a
familiar Queue interface. However, this introduces a dependency from
Format to Queue.

Note, `state.pp_queue` was unnecessarily mutable.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml