use math/rand/v2's fisher-yates impl. pretty sure the one copied from…

… dcrd code introduces bias

crypto/rand/uniform.go

@@ -209,9 +209,18 @@ func (p *PRNG) Duration(n time.Duration) time.Duration {
 // Shuffle randomizes the order of n elements by swapping the elements at
 // indexes i and j.
 func (p *PRNG) Shuffle(n int, swap func(i, j int)) {
-	// Fisher-Yates shuffle.
-	for i := int64(0); i < int64(n); i++ {
-		j := p.Int64n(int64(n)-i) + i
+	if n < 0 {
+		panic("rand: invalid argument to Shuffle")
+	}
+
+	// Fisher-Yates shuffle: https://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle
+	// Shuffle really ought not be called with n that doesn't fit in 32 bits.
+	// Not only will it take a very long time, but with 2³¹! possible permutations,
+	// there's no way that any PRNG can have a big enough internal state to
+	// generate even a minuscule percentage of the possible permutations.
+	// Nevertheless, the right API signature accepts an int n, so handle it as best we can.
+	for i := n - 1; i > 0; i-- {
+		j := int(p.Uint64n(uint64(i + 1)))
 		swap(int(i), int(j))
 	}
 }