Add to/of_bytes functions and support OCaml 4.04.0

• Add Cstruct.of_bytes/to_bytes. In common with the existing
  implementation, this relies on the representation of bytes and string
  being the same, which is true as of OCaml 4.04 and lower (#105 via @yallop).
• Support OCaml 4.04 (#111 via @gasche).