Support OxCaml layouts

[shonfeder]

Odoc doesn't currently show the : layout part of type declarations:

type immediate_kind : immediate
type value_kind : value
type ('a : any) parameterised
type ('a : value, 'b : immediate) parameterised2
type ('a : value) or_null : value_or_null

module Kind (A : sig type t : immediate end) : sig end

Nor the modal bounds, with bounds, and non-modal bounds that go with it:

type immediate_mod_non_float : immediate mod non_float

type 'a list : value mod contended portable many immutable stateless forkable unyielding with 'a

type 'a list_immutable_data : immutable_data with 'a
type 'a imm_external : immutable_data external_

Specified by @art-w

PR #1410