array_unshift: Add fast optimized case for packed arrays

Packed arrays are likely common in this case, as with array_shift which
already has a similar optimization.

For the following benchmark:
```php
<?php

for ($i = 0; $i < 10000000; $i++) {
    $a = [0, 1, 2, 3, 4, 5];
    array_unshift($a, -3, -2, -1);
}
```

On an i7-4790:
```
Benchmark 1: ./sapi/cli/php x.php
  Time (mean ± σ):     753.8 ms ±  23.8 ms    [User: 749.8 ms, System: 2.1 ms]
  Range (min … max):   734.3 ms … 818.6 ms    10 runs

Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):     972.5 ms ±   5.0 ms    [User: 968.8 ms, System: 1.4 ms]
  Range (min … max):   967.8 ms … 984.3 ms    10 runs

Summary
  ./sapi/cli/php x.php  ran
    1.29 ± 0.04 times faster than ./sapi/cli/php_old x.php
```

Author: ndossche

ext/standard/array.c

@@ -3743,18 +3743,34 @@ PHP_FUNCTION(array_unshift)
 	ZEND_PARSE_PARAMETERS_END();
 
 	zend_hash_init(&new_hash, zend_hash_num_elements(Z_ARRVAL_P(stack)) + argc, NULL, ZVAL_PTR_DTOR, 0);
-	for (uint32_t i = 0; i < argc; i++) {
-		Z_TRY_ADDREF(args[i]);
-		zend_hash_next_index_insert_new(&new_hash, &args[i]);
-	}
 
-	ZEND_HASH_FOREACH_STR_KEY_VAL(Z_ARRVAL_P(stack), key, value) {
-		if (key) {
-			zend_hash_add_new(&new_hash, key, value);
-		} else {
-			zend_hash_next_index_insert_new(&new_hash, value);
+	if (HT_IS_PACKED(Z_ARRVAL_P(stack))) {
+		zend_hash_real_init_packed(&new_hash);
+
+		ZEND_HASH_FILL_PACKED(&new_hash) {
+			for (uint32_t i = 0; i < argc; i++) {
+				Z_TRY_ADDREF(args[i]);
+				ZEND_HASH_FILL_ADD(&args[i]);
+			}
+
+			ZEND_HASH_PACKED_FOREACH_VAL(Z_ARRVAL_P(stack), value) {
+				ZEND_HASH_FILL_ADD(value);
+			} ZEND_HASH_FOREACH_END();
+		} ZEND_HASH_FILL_END();
+	} else {
+		for (uint32_t i = 0; i < argc; i++) {
+			Z_TRY_ADDREF(args[i]);
+			zend_hash_next_index_insert_new(&new_hash, &args[i]);
 		}
-	} ZEND_HASH_FOREACH_END();
+
+		ZEND_HASH_FOREACH_STR_KEY_VAL(Z_ARRVAL_P(stack), key, value) {
+			if (key) {
+				zend_hash_add_new(&new_hash, key, value);
+			} else {
+				zend_hash_next_index_insert_new(&new_hash, value);
+			}
+		} ZEND_HASH_FOREACH_END();
+	}
 
 	if (UNEXPECTED(HT_HAS_ITERATORS(Z_ARRVAL_P(stack)))) {
 		zend_hash_iterators_advance(Z_ARRVAL_P(stack), argc);