Interoperability with the integers package

[yallop]

For easy interoperability it would be helpful to have the following type equalities in the interface:

module Usane :
sig
  module Uint8 : 
  sig
    type t = Unsigned.UInt8.t
    (* ... *)
  end

  module Uint16 : 
  sig
    type t = Unsigned.UInt16.t
    (* ... *)
  end

  module Uint32 : 
  sig
    type t = Unsigned.UInt32.t
    (* ... *)
  end

  module Uint64 : 
  sig
    type t = Unsigned.UInt64.t
    (* ... *)
  end
end

rather than the current situation:

module Usane :
sig
  module Uint8 : 
  sig
    type t = int
    (* ... *)
  end

  module Uint16 : 
  sig
    type t = int
    (* ... *)
  end

  module Uint32 : 
  sig
    type t = int32
    (* ... *)
  end

  module Uint64 : 
  sig
    type t = int64
    (* ... *)
  end
end

Besides interoperability, there'd be an added bonus: polymorphic comparison (Pervasives.compare, Pervasives.(<)) would work as expected on Uint32.t and Uint64.t. At present, large positive numbers of those types are represented as negative numbers, and so comparison behaves unexpectedly.

Previous discussion: yallop/ocaml-integers#2