more sugar to be like unicrypto by functions

src/commonTest/kotlin/PBKDTest.kt

@@ -3,6 +3,7 @@ import net.sergeych.crypto2.PBKD
 import net.sergeych.crypto2.initCrypto
 import kotlin.test.Test
 import kotlin.test.assertEquals
+import kotlin.test.assertNotNull
 
 class PBKDTest {
 
@@ -32,5 +33,17 @@ class PBKDTest {
             assertEquals(i.kdp, kx.id.kdp)
 
         }
+        val (y1,y2,y3) = k1.id.kdp!!.kdf.deriveMultiple("foobar", 3)
+        for( (a,b) in listOf(y1,y2,y3).zip(listOf(k1,k2,k3))) {
+            assertEquals(a,b)
+            assertNotNull(a.id.kdp)
+            assertNotNull(b.id.kdp)
+            assertEquals(a.id.kdp, b.id.kdp)
+            assertEquals(a.id, b.id)
+        }
+        assertEquals(k1, y1)
+        assertEquals(k2, y2)
+        assertEquals(k3, y3)
+
     }
 }