0.4.3

CHANGES:

• Add ?opam_version to Dir_context.std_env (@emillon #36).
  It defaults to the version of opam libraries, but in some cases (e.g.
  ocaml-ci) it is useful to inject a value that comes from an external
  opam process.

• Sort Reject after RealImpl (@emillon #33).
  This improves error messages by displaying Rejects first.

• Expose diagnostics rolemap in Solver (@NathanReb #31).
  Allows library users to provide extra help on error.

• Cmdliner 1.1.0 compatibility (@dra27 #40)

• Fix compiler warnings from new fmt (@talex5 #32).