[Cache][String] Don't use is_iterable() in perf-critical code paths

Author: nicolas-grekas

AbstractString.php

@@ -558,7 +558,7 @@ abstract public function trimEnd(string $chars = " \t\n\r\0\x0B\x0C\u{A0}\u{FEFF
      */
     public function trimPrefix($prefix): static
     {
-        if (is_iterable($prefix)) {
+        if (\is_array($prefix) || $prefix instanceof \Traversable) { // don't use is_iterable(), it's slow
             foreach ($prefix as $s) {
                 $t = $this->trimPrefix($s);
 
@@ -592,7 +592,7 @@ abstract public function trimStart(string $chars = " \t\n\r\0\x0B\x0C\u{A0}\u{FE
      */
     public function trimSuffix($suffix): static
     {
-        if (is_iterable($suffix)) {
+        if (\is_array($suffix) || $suffix instanceof \Traversable) { // don't use is_iterable(), it's slow
             foreach ($suffix as $s) {
                 $t = $this->trimSuffix($s);