Add StreamCipher interface and a NopStreamCipher implementation

This will allow us to replace IsaacRandom in the future Netty
encoders/decoders with an implementation suitable for use in a unit
test.

Signed-off-by: Graham <gpe@openrs2.org>

crypto/src/main/kotlin/org/openrs2/crypto/IsaacRandom.kt

@@ -1,6 +1,6 @@
 package org.openrs2.crypto
 
-public class IsaacRandom {
+public class IsaacRandom : StreamCipher {
     private var count = 0
     private val rsl: IntArray
     private val mem = IntArray(SIZE)
@@ -257,7 +257,7 @@ public class IsaacRandom {
         this.a = a
     }
 
-    public fun nextInt(): Int {
+    public override fun nextInt(): Int {
         if (count-- == 0) {
             isaac()
             count = SIZE - 1

crypto/src/main/kotlin/org/openrs2/crypto/NopStreamCipher.kt

@@ -0,0 +1,7 @@
+package org.openrs2.crypto
+
+public object NopStreamCipher : StreamCipher {
+    override fun nextInt(): Int {
+        return 0
+    }
+}

crypto/src/main/kotlin/org/openrs2/crypto/StreamCipher.kt

@@ -0,0 +1,5 @@
+package org.openrs2.crypto
+
+public interface StreamCipher {
+    public fun nextInt(): Int
+}