Fixes and additional APIs

• wrap hexdump_pp output in a box (#175 by @cfcs)
• remove dependency on ocplib-endian (#177 by @hannesm)
• add of_hex: string -> t (#179 by @hannesm and @pqwy)
• add to_bytes: t -> Bytes.t (#183 by @hannesm)
• add empty: t (#184 by @hannesm)
• sub: check the bounds more carefully for overflow (#185 by @hannesm)
• cstruct-unix: fix the build on OCaml 4.06.0 (#187 by @djs55)
• travis: test OCaml 4.04.2 and 4.06.0 (#186 by @hannesm)