Improve performance of array_walk()

We never need to use refcounted copies for arguments because the copy to
the call frame already increments the refcount.

For the following benchmark:
```php
$a = range(0, 10);
for ($i = 0; $i < 1000000; $i++)
    array_walk($a, fn ($val, $idx, $arg) => $val + $arg, 2);
```

On an i7-4790:
```
Benchmark 1: ./sapi/cli/php x.php
  Time (mean ± σ):     593.0 ms ±   5.4 ms    [User: 589.8 ms, System: 1.7 ms]
  Range (min … max):   583.3 ms … 600.9 ms    10 runs

Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):     637.8 ms ±   4.6 ms    [User: 633.9 ms, System: 2.2 ms]
  Range (min … max):   633.4 ms … 649.2 ms    10 runs

Summary
  ./sapi/cli/php x.php ran
    1.08 ± 0.01 times faster than ./sapi/cli/php_old x.php
```

On an i7-1185G7:
```
Benchmark 1: ./sapi/cli/php x.php
  Time (mean ± σ):     362.3 ms ±   2.0 ms    [User: 359.9 ms, System: 1.9 ms]
  Range (min … max):   359.6 ms … 367.1 ms    10 runs

Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):     385.5 ms ±   1.8 ms    [User: 383.2 ms, System: 1.9 ms]
  Range (min … max):   381.5 ms … 387.2 ms    10 runs

Summary
  ./sapi/cli/php x.php ran
    1.06 ± 0.01 times faster than ./sapi/cli/php_old x.php
```

Author: ndossche

ext/standard/array.c

@@ -1457,7 +1457,7 @@ static zend_result php_array_walk(
 	/* Set up known arguments */
 	ZVAL_UNDEF(&args[1]);
 	if (userdata) {
-		ZVAL_COPY(&args[2], userdata);
+		ZVAL_COPY_VALUE(&args[2], userdata);
 	}
 
 	fci.retval = &retval;
@@ -1531,21 +1531,16 @@ static zend_result php_array_walk(
 			}
 			zval_ptr_dtor(&ref);
 		} else {
-			ZVAL_COPY(&args[0], zv);
+			ZVAL_COPY_VALUE(&args[0], zv);
 
 			/* Call the userland function */
 			result = zend_call_function(&fci, &context->fci_cache);
 			if (result == SUCCESS) {
 				zval_ptr_dtor(&retval);
 			}
-
-			zval_ptr_dtor(&args[0]);
 		}
 
-		if (Z_TYPE(args[1]) != IS_UNDEF) {
-			zval_ptr_dtor(&args[1]);
-			ZVAL_UNDEF(&args[1]);
-		}
+		zval_ptr_dtor_str(&args[1]);
 
 		if (result == FAILURE) {
 			break;
@@ -1565,9 +1560,6 @@ static zend_result php_array_walk(
 		}
 	} while (!EG(exception));
 
-	if (userdata) {
-		zval_ptr_dtor(&args[2]);
-	}
 	zend_hash_iterator_del(ht_iter);
 	return result;
 }