Check that constant identifiers actually denote constants
Check that constant identifiers actually denote constants

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