v6.0.0

CHANGES:

breaking changes

• Add the function Cstruct.length, Cstruct.len will
  be deprecated to next release (@dinosaure, @hannesm, @avsm, #279)

• Cstruct.to_string requires 2 optional arguments ?off and ?len
  (@hannesm, @cfcs, @talex5, @dinosaure, #262)

• Use ppxlib instead ppx_tools_versioned
  (@pveber, @avsm, @dinosaure, #280)

• Delete the support of old version of OCaml (< 4.07.0) for ppx_cstruct
  (@avsm, @dinosaure, #280)

• Add useful functions to be able to parse some contents
  with Cstruct.t like the astring library (#227,
  @dinosaure, @avsm, @samoht, @hannesm).

• Improve the interface of Cstruct.t with capabilities
  (@hannesm, @cfcs, @talex5, @dinosaure, #262)