fun shrink(failure: List<T>): List<List<T>>
Given a value, T, this function returns possible "smaller" values to be used as candidates for shrinking.
A smaller value is defined by the Shrinker. For a string it may be considered a "simpler" string (one with less duplication), and/or one with less characters. For an integer it is typically considered a smaller value with a positive sign.
If the value cannot be shrunk further, or the type does not support meaningful shrinking, then this function should return an empty list.