make Keccak an open class

src/commonMain/kotlin/com/infendro/hash/keccak/Keccak.kt

@@ -6,11 +6,16 @@ import com.infendro.bytes.long.copyInto
 import com.infendro.hash.HashFunction
 import com.infendro.hash.util.ceil
 
-abstract class Keccak(
+open class Keccak(
     override val bytes: Int,
     private val rate: Int,
     private val domain: Byte,
 ) : HashFunction {
+    init {
+        require(bytes > 0)
+        require(rate in 8..192 step 8)
+    }
+
     override val block: Int
         get() = rate
 
@@ -64,10 +69,12 @@ abstract class Keccak(
 
     private fun squeeze(state: LongArray, b: LongArray, c: LongArray, d: LongArray, destination: ByteArray) {
         val buffer = ByteArray(8)
+        val threshold = rate / 8
+
         var consumed = 0
         var i = 0
         while (consumed < bytes) {
-            if (i == rate / 8) {
+            if (i == threshold) {
                 permute(state, b, c, d)
                 i = 0
             }