Fix saturating overflow in ExtraData parsing

- Fix overflow in ExtraData by casting byte_len to usize before
multiplication. This was previously multiplying a u16 with
saturating_mul and would overflow when the extra data in string
tables reaches a certain size.
- Add test case for parsing large string table to verify correct
stream position after read.

src/demo/packet/stringtable.rs

@@ -154,7 +154,7 @@ fn test_string_table_roundtrip() {
 #[serde(bound(deserialize = "'a: 'static"))]
 pub struct ExtraData<'a> {
     pub byte_len: u16,
-    #[size = "byte_len.saturating_mul(8)"]
+    #[size = "(byte_len as usize).saturating_mul(8)"]
     pub data: Stream<'a>,
 }