Long module types and types need better line breaks

[lpw25]

The line wrapping for long types and module types makes them very hard to read. Even types that aren't that large (e.g. here and here) are much harder to parse in the docs than they are in their sources.

[Julow]

This is fixed with #782, which works for every type expressions:

val binary_search : 
  ?pos:int ->
  ?len:int ->
  'a t ->
  compare:( 'a -> 'key -> int ) ->
  [ `Last_strictly_less_than
  | `Last_less_than_or_equal_to
  | `Last_equal_to
  | `First_equal_to
  | `First_greater_than_or_equal_to
  | `First_strictly_greater_than ] ->
  'key ->
  int option

type ('a, 'key) f =
  ?pos:int ->
  ?len:int ->
  'a t ->
  compare:( 'a -> 'key -> int ) ->
  [ `Last_strictly_less_than
  | `Last_less_than_or_equal_to
  | `Last_equal_to
  | `First_equal_to
  | `First_greater_than_or_equal_to
  | `First_strictly_greater_than ] ->
  'key ->
  int option

type u =
  < foo : [ `foo | `bar | `baz ]
  ; bar : [ `foo | `bar | `baz ]
  ; baz : [ `foo | `bar | `baz ] >

The next step is to improve the rendering in the HTML backend of type declarations with comments in them: #614, #673