Test that attempting to update a sealed struct is treated as an error.
Test that attempting to update a sealed struct is treated as an error.

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