break-struct for sig fix

[EmileTrotignon]

There was a bug in my #2682 PR to apply break-struct=natural to sig ... end.

This fixes that bug. The bug was that the box structure did not put the end on a new line:

module type T = sig
  type a
  type b end

The implem is a bit hacking, but I tried modifying the box structure to do something cleaner, but it was very hard to do without making diffs all over the place. I can't just copy the struct ... end boxes because of the following:

module S : sig
  type a
  type b
end = struct
  type a = int
  type b = string
end

The code for struct ... end simply handles less cases.

[EmileTrotignon]

No changelog because the bug was introduced in #2682 which is still unreleased.