Add a representation for function declarations.
Add a representation for function declarations.

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