Optionally report stats about fuzz tests (generation, shrinking, rejections)

[Janiczek]

Related to #94, which is about classifying generated values and showing stats about those classes.

It might be nice to show stats unrelated to classifications too: for example, here's a doc on how Hypothesis does things: there, if you run with cmdline argument --hypothesis-show-statistics, you'll see:

- during generate phase (0.06 seconds):
    - Typical runtimes: < 1ms, ~ 47% in data generation
    - 100 passing examples, 0 failing examples, 0 invalid examples
- Stopped because settings.max_examples=100

or, if you use something that throws out a value with invalid (eg. filter, andThen, ...),

test_even_integers:

  - during generate phase (0.08 seconds):
      - Typical runtimes: < 1ms, ~ 57% in data generation
      - 100 passing examples, 0 failing examples, 12 invalid examples
      - Events:
        * 51.79%, Retried draw from integers().filter(lambda x: x % 2 == 0) to satisfy filter
        * 10.71%, Aborted test because unable to satisfy integers().filter(lambda x: x % 2 == 0)
  - Stopped because settings.max_examples=100

or, if you use an event function to classify a generated value,

test_even_integers:

  - during generate phase (0.09 seconds):
      - Typical runtimes: < 1ms, ~ 59% in data generation
      - 100 passing examples, 0 failing examples, 32 invalid examples
      - Events:
        * 54.55%, Retried draw from integers().filter(lambda x: x % 2 == 0) to satisfy filter
        * 31.06%, i mod 3 = 2
        * 28.79%, i mod 3 = 0
        * 24.24%, Aborted test because unable to satisfy integers().filter(lambda x: x % 2 == 0)
        * 15.91%, i mod 3 = 1
  - Stopped because settings.max_examples=100