Dock 'object .. end' blocks in class signatures

[Julow]

This includes #2297
Fix #2288

Make class signatures more consistent with class structures. This does not make class signature consistent with function signatures yet but is a step in the right direction.

[gpetiot]

Great job!

Do you think we could slightly improve the formatting of class c : 'a -> let open X in k for the Jane Street profile? We get:

class c : 'a
  -> let open X in
     k

Before the change it was formatted as:

class c :
  'a
  -> let open X in
     k

With the conventional profile we get:

class c : 'a ->
  let open X in
  k

[Julow]

Considering that not everything can go after a let open, there can be a better formatting. I'd prefer not to block this PR too long though.

[gpetiot]

Fair enough! All good to go then!