Optimize stripcslashes

See twigphp#4176 (comment)

Co-Authored-By: Simon André <[email protected]>

```
# Old
Benchmark 1: php bench.php
  Time (mean ± σ):      1.192 s ±  0.004 s    [User: 1.163 s, System: 0.009 s]
  Range (min … max):    1.186 s …  1.198 s    10 runs

# New
Benchmark 1: php bench.php
  Time (mean ± σ):     356.5 ms ±   1.1 ms    [User: 342.0 ms, System: 7.6 ms]
  Range (min … max):   354.6 ms … 358.2 ms    10 runs
```

Using bench.php:
```
<?php

require_once __DIR__.'/vendor/autoload.php';

$string = str_repeat("Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut a tincidunt turpis. \\App\\Entity\\Product \World \065 App\#{var} \'quoted\' Donec pharetra enim quis erat pharetra, dignissim molestie erat laoreet. Vivamus auctor purus sed lorem vestibulum rhoncus.", 10);
$loader = new Twig\Loader\ArrayLoader([
    'index.twig' => <<<EOF
{{ "$string" }}
EOF
    ,
]);

$twig = new Twig\Environment($loader);

for ($i = 0; $i < 10000; $i++) {
    $twig->tokenize(new Twig\Source($twig->getLoader()->getSourceContext('index.twig')->getCode(), 'index.twig'));
}

```

src/Lexer.php

@@ -398,62 +398,55 @@ private function stripcslashes(string $str, string $quoteType): string
             'v' => "\v",
         ];
 
-        for ($i = 0; $i < $length; $i++) {
-            if ($str[$i] !== '\\' || $i + 1 >= $length) {
-                $result .= $str[$i];
+        $i = 0;
+        while ($i < $length) {
+            if (false === $pos = strpos($str, '\\', $i)) {
+                $result .= substr($str, $i);
+                break;
+            }
 
-                continue;
+            $result .= substr($str, $i, $pos - $i);
+            $i = $pos + 1;
+
+            if ($i >= $length) {
+                $result .= '\\';
+                break;
             }
 
-            $nextChar = $str[$i + 1];
+            $nextChar = $str[$i];
+
             if (isset($specialChars[$nextChar])) {
                 $result .= $specialChars[$nextChar];
-                $i++;
             } elseif ($nextChar === '\\') {
-                $result .= '\\';
-                $i++;
+                $result .= $nextChar;
             } elseif ($nextChar === "'" || $nextChar === '"') {
                 if ($nextChar !== $quoteType) {
-                    trigger_deprecation('twig/twig', '3.12', 'Character "%s" at position %d does not need to be escaped anymore.', $nextChar, $i + 2);
+                    trigger_deprecation('twig/twig', '3.12', 'Character "%s" at position %d does not need to be escaped anymore.', $nextChar, $i + 1);
                 }
                 $result .= $nextChar;
-                $i++;
-            } elseif ($nextChar === '#' && $i + 2 < $length && $str[$i + 2] === '{') {
+            } elseif ($nextChar === '#' && $i + 1 < $length && $str[$i + 1] === '{') {
                 $result .= '#{';
-                $i += 2;
-            } elseif ($nextChar === 'x' && $i + 2 < $length && ctype_xdigit($str[$i + 2])) {
-                $hex = $str[$i + 2];
-                if ($i + 3 < $length && ctype_xdigit($str[$i + 3])) {
-                    $hex .= $str[$i + 3];
-                    $i++;
+                $i++;
+            } elseif ($nextChar === 'x' && $i + 1 < $length && ctype_xdigit($str[$i + 1])) {
+                $hex = $str[++$i];
+                if ($i + 1 < $length && ctype_xdigit($str[$i + 1])) {
+                    $hex .= $str[++$i];
                 }
                 $result .= chr(hexdec($hex));
-                $i += 2;
             } elseif (ctype_digit($nextChar) && $nextChar < '8') {
                 $octal = $nextChar;
-                for ($j = $i + 1; $j <= 3; $j++) {
-                    $o = $j + 1;
-                    if ($o >= $length) {
-                        break;
-                    }
-                    if (!ctype_digit($str[$o])) {
-                        break;
-                    }
-                    if ($str[$o] >= '8') {
-                        break;
-                    }
-                    $octal .= $str[$o];
-                    $i++;
+                while ($i + 1 < $length && ctype_digit($str[$i + 1]) && $str[$i + 1] < '8' && strlen($octal) < 3) {
+                    $octal .= $str[++$i];
                 }
                 $result .= chr(octdec($octal));
-                $i++;
             } else {
-                trigger_deprecation('twig/twig', '3.12', 'Character "%s" at position %d does not need to be escaped anymore.', $nextChar, $i + 2);
+                trigger_deprecation('twig/twig', '3.12', sprintf('Character "%s" at position %d does not need to be escaped anymore.', $nextChar, $i + 1));
                 $result .= $nextChar;
-                $i++;
             }
+
+            $i++;
         }
-        
+
         return $result;
     }