Case folding support for environments without mbstring

Per the spec, reference link matching is done by normalizing the label with a Unicode case fold, which mb_strtoupper provides.

But since not all systems have the relevant extension installed, we need to manually implement this logic by converting characters according to this table: http://www.unicode.org/Public/UNIDATA/CaseFolding.txt

src/Reference/Reference.php

@@ -14,6 +14,8 @@
 
 namespace League\CommonMark\Reference;
 
+use League\CommonMark\Util\UnicodeCaseFolder;
+
 /**
  * Link reference
  */
@@ -87,6 +89,12 @@ public static function normalizeReference($string)
         // leading/trailing whitespace
         $string = preg_replace('/\s+/', '', trim($string));
 
+        // Convert to upper-case using Unicode case folding
+        // Use an alternate method if mb_strtoupper isn't available
+        if (!function_exists('mb_strtoupper')) {
+            return UnicodeCaseFolder::toUpperCase($string);
+        }
+
         return mb_strtoupper($string, 'UTF-8');
     }
 }