Int shrinkers

[jmid]

This is addressing #82.
It just copies the int shrinking strategy. Potentially we could factor out the general strategy and parameterize all three integer shrinkers over this common core. For now I chose to keep them very similar and right next to each other.

It seems to work fine:

# let t = Test.make int32 (fun i -> i < 2343544l);;
val t : Test.t = QCheck.Test.Test <abstr>
# QCheck_runner.run_tests ~verbose:true [t];;
generated error fail pass / total     time test name
[✗]    1    0    1    0 /  100     0.0s anon_test_3

--- Failure --------------------------------------------------------------------

Test anon_test_3 failed (14 shrink steps):

2343544l
================================================================================
failure (1 tests failed, 0 tests errored, ran 1 tests)
- : int = 1
# QCheck_runner.run_tests ~verbose:true [t];;
generated error fail pass / total     time test name
[✗]    1    0    1    0 /  100     0.0s anon_test_3

--- Failure --------------------------------------------------------------------

Test anon_test_3 failed (16 shrink steps):

2343544l
================================================================================
failure (1 tests failed, 0 tests errored, ran 1 tests)
- : int = 1
# let t = Test.make int64 (fun i -> i < 2343544L);;
val t : Test.t = QCheck.Test.Test <abstr>
# QCheck_runner.run_tests ~verbose:true [t];;
generated error fail pass / total     time test name
[✗]    5    0    1    4 /  100     0.0s anon_test_4

--- Failure --------------------------------------------------------------------

Test anon_test_4 failed (51 shrink steps):

2343544L
================================================================================
failure (1 tests failed, 0 tests errored, ran 1 tests)
- : int = 1
# QCheck_runner.run_tests ~verbose:true [t];;
generated error fail pass / total     time test name
[✗]    1    0    1    0 /  100     0.0s anon_test_4

--- Failure --------------------------------------------------------------------

Test anon_test_4 failed (49 shrink steps):

2343544L
================================================================================
failure (1 tests failed, 0 tests errored, ran 1 tests)
- : int = 1