add Arb.slice 

/**
 * Generates a random slice of the input list, including the empty list.
 * The returned list has the same order as the input list. 
 */

also add Exhaustive.slice

```
      test("Exhaustive.slices for strings") {
         Exhaustive.slices(listOf("apple", "orange", "banana")).values shouldContainExactlyInAnyOrder listOf(
            listOf("apple"),
            listOf("apple", "orange"),
            listOf("apple", "orange", "banana"),
            listOf("orange"),
            listOf("orange", "banana"),
            listOf("banana"),
         )
      }
```