v4.0.0

CHANGES:

• Rename the to/from_bytes functions to refer to octets
  instead. This distinguishes the meaning of human-readable
  addresses (strings in this library) and byte-packed
  representations(octets in this library) from the OCaml
  bytes type that represents mutable strings.

  Porting code should just be a matter of renaming functions
  such as:

  • Ipaddr.of_bytes becomes Ipaddr.of_octets
  • Macaddr.to_bytes becomes Macaddr.to_octets

• Introduce new write_octets functions that can write
  octet representations of IPv4/v6 into an existing bytestring.

• Use the domain-name library to produce domain names
  from IP addresses.

• Remove the ipaddr.sexp and macaddr.sexp ocamlfind
  subpackages and instead have ipaddr-sexp and macaddr-sexp
  to match the opam package names.

• Add new Ipaddr_cstruct and Macaddr_cstruct libraries
  for conversion to/from cstructs (#36 @nojb @avsm)