Generate all permutations of the list with equal probability

Generate all permutations of the list with equal probability

@botovq was kind enough to point out that although my earlier
implementation produced random-ish ordering, it strictly speaking
wasn't Fisher-Yates.

We need to ensure `j` is a random number between `i` and `list.count`
see the second example in the 'Modern Algorithm'
https://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle