OpenHashMap.mergeX: Avoid triple-hashing key

Previously, we were hashing the key 3x:
- get
- containsKey
- put

Now, we override mergeDouble with a hashamp-specific implementation that
hashes the key once.

Towards vigna#336

drv/OpenHashMap.drv

@@ -1253,6 +1253,25 @@ public class OPEN_HASH_MAP KEY_VALUE_GENERIC extends ABSTRACT_MAP KEY_VALUE_GENE
 		return value[pos] = newVal;
 	}
 
+#if VALUES_PRIMITIVE && ! VALUE_CLASS_Boolean
+	/** {@inheritDoc} */
+	@Override
+	public VALUE_GENERIC_TYPE MERGE_VALUE(final KEY_GENERIC_TYPE k, final VALUE_GENERIC_TYPE v, METHOD_ARG_VALUE_BINARY_OPERATOR remappingFunction) {
+		java.util.Objects.requireNonNull(remappingFunction);
+		REQUIRE_VALUE_NON_NULL(v)
+
+		final int pos = find(k);
+		if (pos < 0) {
+			insert(-pos - 1, k, v);
+			return v;
+		}
+
+		final VALUE_GENERIC_TYPE newValue = remappingFunction.VALUE_OPERATOR_APPLY(value[pos], v);
+
+		return value[pos] = newValue;
+	}
+#endif
+
 	/** {@inheritDoc} */
 	@Override
 	public VALUE_GENERIC_TYPE merge(final KEY_GENERIC_TYPE k, final VALUE_GENERIC_TYPE v, final java.util.function.BiFunction<? super VALUE_GENERIC_CLASS, ? super VALUE_GENERIC_CLASS, ? extends VALUE_GENERIC_CLASS> remappingFunction) {

test/it/unimi/dsi/fastutil/objects/Object2IntOpenHashMapTest.java

@@ -227,12 +227,13 @@ public int hashCode() {
 		}
 	}
 
+	// Regression test for https://github.com/vigna/fastutil/pull/337.
 	@Test
-	public void testMergeIntHashesKeyMultipleTimes() {
+	public void testMergeIntHashesKeyOnce() {
 		Object2IntOpenHashMap m = new Object2IntOpenHashMap(Hash.DEFAULT_INITIAL_SIZE);
 		HashCounter hc = new HashCounter();
 		m.mergeInt(hc, 0, Math::max);
-		assertEquals(3, hc.hashCount);
+		assertEquals(1, hc.hashCount);
 	}
 }