protocol_compiler: split the large functor into chunks

Split the large functor into chunks

Transform:

module Make(P : PROTOCOL_VERSION) = struct
  open P
  open DEFAULT

  module M1 = ...
  ...
  module Mn = ...
end

into:

open struct module Make(P : PROTOCOL_VERSION) = struct
  open P
  open DEFAULT

  module M1 = ...
  ...
  module Mi = ...
end end

module Make(P : PROTOCOL_VERSION) = struct
  open P
  open DEFAULT

  include Make (P)
  module Mi+1 = ...
  ...
  module Mn = ...
end

The patch was written by @let-def.