[#401] internal: Place randomization code in random package

Also replace seeding into `init` function.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

internal/random/rand.go

@@ -0,0 +1,15 @@
+package random
+
+import (
+	"math/rand"
+	"time"
+)
+
+func init() {
+	rand.Seed(time.Now().UnixNano())
+}
+
+// Uint32 returns random uint32 value [0, max).
+func Uint32(max uint32) uint32 {
+	return rand.Uint32() % max
+}