standard: Optimize str_split()

Three optimizations:
- If the entire string is returned, we don't need to duplicate it.
- Use packed filling logic.
- Use fast construction of strings. This is useful when splitting
  strings on length=1. In that case I get a 6x speedup in the code
  below.

Bench:
```php
$x = str_repeat('A', 100);
for ($i = 0; $i < 1000000; $i++)
    str_split($x, 10);
```

On an i7-4790:
```
Benchmark 1: ./sapi/cli/php x.php
  Time (mean ± σ):     160.1 ms ±   6.4 ms    [User: 157.3 ms, System: 1.8 ms]
  Range (min … max):   155.6 ms … 184.7 ms    18 runs

Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):     202.6 ms ±   4.0 ms    [User: 199.1 ms, System: 1.9 ms]
  Range (min … max):   197.4 ms … 209.2 ms    14 runs

Summary
  ./sapi/cli/php x.php  ran
    1.27 ± 0.06 times faster than ./sapi/cli/php_old x.php
```

The performance gain increases with smaller lengths.

Author: ndossche

ext/standard/string.c

@@ -6152,23 +6152,31 @@ PHP_FUNCTION(str_split)
 		}
 
 		array_init_size(return_value, 1);
-		add_next_index_stringl(return_value, ZSTR_VAL(str), ZSTR_LEN(str));
+		GC_TRY_ADDREF(str);
+		add_next_index_str(return_value, str);
 		return;
 	}
 
 	array_init_size(return_value, (uint32_t)(((ZSTR_LEN(str) - 1) / split_length) + 1));
+	zend_hash_real_init_packed(Z_ARRVAL_P(return_value));
 
 	n_reg_segments = ZSTR_LEN(str) / split_length;
 	p = ZSTR_VAL(str);
 
-	while (n_reg_segments-- > 0) {
-		add_next_index_stringl(return_value, p, split_length);
-		p += split_length;
-	}
+	ZEND_HASH_FILL_PACKED(Z_ARRVAL_P(return_value)) {
+		zval zv;
+		while (n_reg_segments-- > 0) {
+			ZEND_ASSERT(split_length > 0);
+			ZVAL_STRINGL_FAST(&zv, p, split_length);
+			ZEND_HASH_FILL_ADD(&zv);
+			p += split_length;
+		}
 
-	if (p != (ZSTR_VAL(str) + ZSTR_LEN(str))) {
-		add_next_index_stringl(return_value, p, (ZSTR_VAL(str) + ZSTR_LEN(str) - p));
-	}
+		if (p != (ZSTR_VAL(str) + ZSTR_LEN(str))) {
+			ZVAL_STRINGL_FAST(&zv, p, (ZSTR_VAL(str) + ZSTR_LEN(str) - p));
+			ZEND_HASH_FILL_ADD(&zv);
+		}
+	} ZEND_HASH_FILL_END();
 }
 /* }}} */