Add serialization of blocks with Ring CT signatures.

Currently the serialization/deserialization of [`Block`][2]s with Ring CT signatures are [unimplemented][1].

[1]: https://github.com/xmr-rs/xmr/blob/470098ff7d105c390473190cab1c9ca3b789f3ea/chain/src/transaction/transaction.rs#L30
[2]: https://github.com/xmr-rs/xmr/blob/master/chain/src/block.rs