feat: implement tuple trait impls and flatten field attribute

[stevencartavia]

Closes #14

Implements the remaining items from the new library and macro API:

• RlpEncodable/RlpDecodable impls for tuples (arity 1–12), encoding as RLP lists
• #[rlp(flatten)] field attribute using rlp_encode_raw/rlp_decode_raw/rlp_len_raw
• Tagged enum trailing-bytes validation
• Compile-time rejection of #[rlp(flatten)] on Option<T> fields
• Saturating arithmetic in Decoder::bytepos
• Sentinel-aware optional decoding in rlp_decode_raw

[DaniPopes]

these should likely become methods on deserializer that keeps track of the position and does like return Err(self.mk_error()) so that the position is propagated

[stevencartavia]

Added decode_header, decode_bytes, and decode_str on Decoder with position-aware errors. Moving the parsing logic itself would require changing the RlpDecodable::rlp_decode signature from &mut &[u8] to &mut Decoder.