:: ignores single- vs multi-line

[Niols]

As of 8cc9aa4, Topiary formats:

let () = a :: b
    :: c

as:

let () =
  a :: b :: c

which takes into account the multi-line mode for let but not for ::. I would expect it to be multi-line, eg:

let () =
  a ::
    b ::
    c

or

let () =
  a
    :: b
    :: c

Related to #657 for uniformity of the handling of operators.

To reproduce:

$ nix run github:tweag/topiary/8cc9aa40d -- format -l ocaml <<EOF
let () = a :: b
    :: c
EOF