Optimize list shrinking for "any list over count" situation

[nth-commit]

A reasonably common counterexample might due to a list of over n count, or a string over n length.

However, the smallest example that satisfies this predicate is actually quite hard to get to, as we focus on bisecting the list whilst shrinking the elements first.

Consider the property:

public void StringProp(string s)
{
    Xunit.Assert.True(s.Length < 2);
}

Takes approximately 20 shrinks to find the smallest countexample ("aa"). It also does not scale linearly:

Length	~Number of shrinks
2	20
4	60
8	180
16	500
32	1500

We should prioritize finding the smallest example that satisfies that constraint. That is, the first shrink of a list should be shrinking all the elements to their smallest form.