encoder/filename: Add SCSU as tables

Instead of only adding SCSU, add it as an existing table.

Allow direct SCSU and add a, perhaps, reasonable table as well.

Add byte interfaces that doesn't base64 encode the URL as well with `EncodeBytes` and `DecodeBytes`.

Fuzz tested and decode tests added.

lib/encoder/filename/fuzz.go

@@ -25,7 +25,9 @@ func Fuzz(data []byte) int {
 		panic(fmt.Sprintf("error decoding %q, input %q: %v", enc, string(data), err))
 	}
 	if !bytes.Equal(data, []byte(decoded)) {
-		panic(fmt.Sprintf("decode mismatch, encoded: %q, org: %q, got: %q", enc, string(data), decoded))
+		table := decodeMap[enc[0]]
+		table--
+		panic(fmt.Sprintf("decode mismatch, encoded: %q, org: %q, got: %q, table %d", enc, string(data), decoded, int(table)))
 	}
 
 	// Everything is good.