0.4.0

CHANGES:

• Drop OCaml 4.08 to 4.10, which opam-repository no longer carries: alcobar
  now requires OCaml 4.11 or later (#4, @samoht)
• Add test_case ~outcomes and reach: name the cases a property must
  exercise, see how often each was hit, and fail when one never is, so a
  generator that silently stops covering a case is caught (@samoht)
• Add verdict, a check over the whole campaign (such as "every outcome was
  reached") run once after all properties; AFL never feeds it inputs, so a
  campaign-wide check no longer turns into a fuzzing crash (#3, @samoht)
• Fix --gen-corpus for suites with several tests: each seed now replays the
  test it was made for, so afl-cmin accepts the corpus instead of reporting
  "No instrumentation detected"; regenerate a corpus written by 0.3 (#3,
  @samoht)
• --gen-corpus DIR is a command-line option: --help lists it, it takes
  --gen-corpus=DIR, and a missing directory is refused by name instead of
  reading as an unknown option (#6, @samoht, reported by @patricoferris in #2)
• The README shows the dune rules to fuzz a test with afl-fuzz (#6, @samoht,
  reported by @patricoferris in #2)