v4.0.0

CHANGES:

• Sexplib is now an optional library for the base Cstruct module.
  A new Cstruct_sexp module has been introduced with the serialiser
  functions, contained within the cstruct-sexp opam package.

  To convert old code, simply use Cstruct_sexp.t instead of
  Cstruct.t in a record type for which you are using [@@deriving sexp].
  This is a type alias to Cstruct.t but also has the right
  sexp-conversion functions in scope. There is an example of this
  in the ppx_test/with-sexp directory in the source repo.

  When you have converted and released your library, add an
  opam constraint of cstruct {>="4.0.0"} to your own opam
  packages to ensure that they pick up this version of the library.
  (fixes #222, @avsm)

• JavaScript stubs are now installed using the jsoo mechanism
  rather than a manual specification (#241 @jonludlam)

• Use computed versions in opam files to ensure that dependent
  opam packages such as cstruct-async get the same base version
  of cstruct to avoid mismatches. (@avsm)

• Add a ppx test suite to checks that all error paths in ppx
  handling are reachable and have meaningful location info (#238 @emillon)