`ExitKind::Ok` exposing the result of `libfuzzer_test_one_input`

Currently, `libfuzzer_test_one_input` returns an i32 that is discarded when turning into `ExitKind::Ok`, whereas the value could potentially signal interestingness from the harness leveraged by domain-specific fuzzers. I suppose `ExitKind` is part of the public API so it wouldn't be possible to change it after this point, but perhaps an `ExitKind::Value(i32)` would come in handy?