feat: optimise normalize helper

csharp/PhoneNumbers/PhoneNumberUtil.cs

@@ -869,8 +869,10 @@ public static string GetCountryMobileToken(int countryCallingCode)
         /// should be stripped from the number. If this is false, they
         /// will be left unchanged in the number.</param>
         /// <returns>The normalized string version of the phone number.</returns>
+#if !(NET5_0_OR_GREATER || NETSTANDARD2_1_OR_GREATER)
         private static string NormalizeHelper(string number, Func<char, char> normalizationReplacements, bool removeNonMatches)
             => NormalizeHelper(new StringBuilder(number), normalizationReplacements, removeNonMatches).ToString();
+#endif
 
         private static StringBuilder NormalizeHelper(StringBuilder number, Func<char, char> normalizationReplacements, bool removeNonMatches)
         {

csharp/PhoneNumbers/PhoneNumberUtil.net.cs

@@ -606,6 +606,15 @@ private static void NormalizeHelper(ref Span<char> span,
             }
         }
 
+        private static string NormalizeHelper(string number, Func<char, char> normalizationReplacements, bool removeNonMatches)
+        {
+            if (number.Length == 0) return string.Empty;
+            Span<char> result = stackalloc char[number.Length];
+            var resultLength = 0;
+            NormalizeHelper(ref result, ref resultLength, number, normalizationReplacements, removeNonMatches);
+            return new string(result.Slice(0, resultLength));
+        }
+
         private static bool IsValidAlphaPhone(string number)
         {
             for (int alpha = 0, i = 0; i < number.Length; i++)