Improve handling of discarded tests

[jobjo]

A few things:

1. Add a configuration max_num_discarded for specifying that a property-based test fails if it exceeds the threshold for the number of discarded items.

2. Expose missing Popper.discard function.

3. Improve printing of discarded unit test.

4. Change the number of samples for the test target to 1000.

Example:

let config = Config.max_num_discarded 50
let () =
  check ~config (fun () ->
    let* b = Sample.bool in
    if b then pass else discard)

Results in: