Optimized HexToBytes function (#469)

- used Span
- used Convert.FromHexString vectorized function

src/N_m3u8DL-RE.Common/Util/HexUtil.cs

@@ -34,15 +34,13 @@ namespace N_m3u8DL_RE.Common.Util
 
         public static byte[] HexToBytes(string hex)
         {
-            hex = hex.Trim();
-            if (hex.StartsWith("0x") || hex.StartsWith("0X"))
-                hex = hex.Substring(2);
-            byte[] bytes = new byte[hex.Length / 2];
+            var hexSpan = hex.AsSpan().Trim();
+            if (hexSpan.StartsWith("0x") || hexSpan.StartsWith("0X"))
+            {
+                hexSpan = hexSpan.Slice(2);
+            }
 
-            for (int i = 0; i < hex.Length; i += 2)
-                bytes[i / 2] = Convert.ToByte(hex.Substring(i, 2), 16);
-
-            return bytes;
+            return Convert.FromHexString(hexSpan);
         }
     }
 }