Parentheses in type signatures do not seem to be handled at all

[Niols]

As of f35eee4, Topiary formats:

val foo :
  named_argument: (
    int ->
    float ->
    string
  ) ->
  other_argument ->
  result

as:

val foo :
  named_argument:
  (int ->
  float ->
  string) ->
  other_argument ->
  result

which is just... wrong. I'm not sure my snippet is how Topiary should render it, but at least it should definitely render it how it does it now.

Somewhat related to dangling parentheses: #661

To reproduce:

$ nix run github:tweag/topiary/f35eee4 -- format -l ocaml_interface <<EOF
val foo :
  named_argument: (
    int ->
    float ->
    string
  ) ->
  other_argument ->
  result
EOF

[Niols]

Note that this isn't limited to named arguments (it is just how I discovered it):

val foo :
  (        
    int ->
    float ->
    string
  ) ->
  other_argument ->
  result           

gets formatted as

val foo :
  (int ->
  float ->
  string) ->
  other_argument ->
  result

[Niols]

Also, combo with #816:

val foo :
  named_argument:
    (
      int,
      string
    )
      Hashtbl.t ->
  other_argument ->
  result

gets formatted as

val foo :
  named_argument:
  (int, string) Hashtbl.t ->
  other_argument ->
  result

Again, my snippet doesn't look good so I'm not sure how things should look, but not like this.