QCheck int shrinker produces duplicates

[jmid]

As part of #235 I was reminded of the following - which we should address and track separately:

The duplicate 0 testing of the improved int shrinker is unsatisfying, but so is the duplicate output of the current int shrinker:

qcheck/test/core/QCheck_unit_tests.ml

Lines 20 to 24 in 824dafb

	let test_int () =
	List.iter (alco_check Alcotest.int (trace_false Shrink.int) "on repeated failure")
	[ ("int 100", 100, [50; 75; 88; 94; 97; 99; 99]); (*WTF?*)
	("int 1000", 1000, [500; 750; 875; 938; 969; 985; 993; 997; 999; 999]); (*WTF?*)
	("int (-26)", -26, [-13; -20; -23; -25; -25]) ]; (*WTF?*)

Originally posted by @jmid in #235 (comment)