Type parameters 'a. aren't left on their own line

[Niols]

As of f35eee4, Topiary formats:

let foo :
    'a.
    'a ->
    'a ->
    'a
= fun x y -> x

as:

let foo
  : 'a. 'a ->
  'a ->
  'a
= fun x y -> x

Note how the 'a. gets on the same line as the first lhs of the arrow, suggesting a wrong precedence of . wrt ->. I think the snippet should stay unchanged.

To reproduce:

$ nix run github:tweag/topiary/f35eee4 -- format -l ocaml <<EOF
let foo :
    'a.
    'a ->
    'a ->
    'a
  = fun x y -> x
EOF

Somewhat related: #814