tuple support for Encodable/Decodable trait

In the `serialize` crate, it seems that all implementations of tuples for the `Encodable` and `Decodable` traits are using `{emit,read}_seq` and `{emit,read}_seq_elt`. Should these implementations be using `read_tuple` (and others) instead? (It looks like `read_tuple` is never used.) Otherwise, using `Encodable`/`Decodable` it's impossible to distinguish between tuples and sequences.
