Support for "with module type" constraints

[Octachron]

This PR adds support for with module type constraint:

module type s = sig module type T module M:T end
module type u = s with module type T := s

and local module type substitution:

module type s:= sig
  type t
  type r
end
module M: s

This change adds a new class of fragments for module types which explains the size of the new code.

There is some cases in xref2/subst.ml that would require more attention during review: there are few assertions about which module type paths can be substituted or not.

One limitation of this PR is that deep destructive substitution is not supported. For instance,

module type s = sig
  module M: sig
     module type r
  end
  module N : M.r
end
with module type M.r := sig end

creates a dead link M.r in the constrained signature. However, this is a global issue with all deep destructive substitutions. It seems better to fix this issue independently.

[jonludlam]

So we're saying here that at the point we're substituting A := B then A has to exist? And this is a constraint that comes from the compiler, so while the types in odoc allow it we should never be in this situation?

[jonludlam]

This all looks very sensible indeed. Many thanks!