Failure during include

[jonludlam]

Repro (due to @lukemaurer)

a_intf.ml

module type S = sig
  module Foo : sig end
end

b_intf.ml

module Bar = struct end

module type S = sig
  include A_intf.S
  module Foo : sig end
end

module type B = sig
  module type S = S

  include S
end

b.mli

include B_intf.B

c.mli

include B.S

Causes a failure in Subst

[jonludlam]

Fixed in #603