Optimize ext/dom $wholeText (#15001)

Use our own string builder instead of using libxml's and then having to
copy over.

For the following test:
```
$dom = Dom\HTMLDocument::createEmpty();
$root = $dom->appendChild($dom->createElement('root'));
$root->append('abc', 'def', 'ghi');
$f = $root->firstChild;

for ($i = 0; $i < 1000000; $i++)
	$f->wholeText;
```

The following results were obtained on an i7-4790:
```
Benchmark 1: ./sapi/cli/php x.php
  Time (mean ± σ):      57.2 ms ±   2.3 ms    [User: 53.2 ms, System: 3.4 ms]
  Range (min … max):    54.7 ms …  69.3 ms    52 runs

Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):      89.4 ms ±   3.4 ms    [User: 85.6 ms, System: 3.0 ms]
  Range (min … max):    86.1 ms … 105.8 ms    32 runs

Summary
  ./sapi/cli/php x.php ran
    1.56 ± 0.09 times faster than ./sapi/cli/php_old x.php
```

ext/dom/text.c

@@ -68,7 +68,7 @@ zend_result dom_text_whole_text_read(dom_object *obj, zval *retval)
 {
 	DOM_PROP_NODE(xmlNodePtr, node, obj);
 
-	xmlChar *wholetext = NULL;
+	smart_str str = {0};
 
 	/* Find starting text node */
 	while (node->prev && ((node->prev->type == XML_TEXT_NODE) || (node->prev->type == XML_CDATA_SECTION_NODE))) {
@@ -77,16 +77,11 @@ zend_result dom_text_whole_text_read(dom_object *obj, zval *retval)
 
 	/* concatenate all adjacent text and cdata nodes */
 	while (node && ((node->type == XML_TEXT_NODE) || (node->type == XML_CDATA_SECTION_NODE))) {
-		wholetext = xmlStrcat(wholetext, node->content);
+		smart_str_appends(&str, (const char *) node->content);
 		node = node->next;
 	}
 
-	if (wholetext != NULL) {
-		ZVAL_STRING(retval, (char *) wholetext);
-		xmlFree(wholetext);
-	} else {
-		ZVAL_EMPTY_STRING(retval);
-	}
+	ZVAL_STR(retval, smart_str_extract(&str));
 
 	return SUCCESS;
 }