Optimise ObjectsHashIterative hash function.

Avoid using Arrays.deepHashCode. The array passed to deepHashCode is
always length 2. So we can unroll the same computation for the fixed 2
iterations.

src/main/java/org/apache/commons/collections4/bloomfilter/hasher/function/ObjectsHashIterative.java

@@ -60,7 +60,14 @@ public final class ObjectsHashIterative implements HashFunction {
         if (seed == 0) {
             last = 0;
         }
-        final long result = Arrays.deepHashCode(new Object[] { last, buffer });
+        // Effectively:
+        // result = Arrays.deepHashCode(new Object[] { last, buffer });
+        // The method loops over items starting with result=1
+        // for i in items:
+        //    result = 31 * result + hashCode(i)
+        // Here we unroll the computation to 2 iterations.
+        // The computation is done using 32-bit integers then cast to a long
+        final long result = 31 * (31 + Long.hashCode(last)) + Arrays.hashCode(buffer);
         last += result;
         return result;
     }

src/test/java/org/apache/commons/collections4/bloomfilter/hasher/function/ObjectsHashIterativeTest.java

@@ -41,12 +41,11 @@ public class ObjectsHashIterativeTest extends AbstractHashFunctionTest {
         long l = obj.apply(buffer, 0);
         long prev = 0;
         assertEquals(Arrays.deepHashCode(new Object[] {prev, buffer}), l);
-        prev += l;
-        l = obj.apply(buffer, 1);
-        assertEquals(Arrays.deepHashCode(new Object[] {prev, buffer}), l);
-        prev += l;
-        l = obj.apply(buffer, 2);
-        assertEquals(Arrays.deepHashCode(new Object[] {prev, buffer}), l);
+        for (int i = 1; i <= 5; i++) {
+            prev += l;
+            l = obj.apply(buffer, i);
+            assertEquals(Arrays.deepHashCode(new Object[] {prev, buffer}), l);
+        }
     }
 
     @Override