Add a function for pairing a value with 'errno' to ctypes_cstubs_internals_h.
Add a function for pairing a value with 'errno' to ctypes_cstubs_internals_h.

dsheets/ocaml-ctypes,yallop/ocaml-ctypes,yallop/ocaml-ctypes,ocamllabs/ocaml-ctypes,whitequark/ocaml-ctypes,whitequark/ocaml-ctypes,dsheets/ocaml-ctypes,ocamllabs/ocaml-ctypes,whitequark/ocaml-ctypes,dsheets/ocaml-ctypes