Feature: 'include' statement

[plt-amy]

include is exactly as open except the signature of the included module is concatenated with the signature of the including module. Concretely:

module X : sig val x : int end = begin
  let x = 1
end

module Y = (begin
  let y = 1
  include X
end) : sig
  val x : int
  val y : int
end

[SquidDev]

One tiny thing, but otherwise looks good.

I REJECT MY REVIEW, JOJO!