Implement SSZ

[dangerousfood]

Implement SSZ in accordance with the spec listed here. Additional resources can be found here here

[jrhea]

@dangerousfood What is the status of this? Do you have an implementation in Java, or are you going to use Antoine's version in Cava?

[atoulme]

tagging myself @atoulme so I get notified of updates.

[jrhea]

@atoulme you finished the implementation in cava, right?

[atoulme]

I would have to look again. The spec changed a bit since. I was unsure if we needed a special construct to handle containers. There were changes related to endianness. I also had left out the hashing.

[jrhea]

Just glanced at the cava source and it looks like one potential issue with using the cava SSZ implementation is:

import net.consensys.cava.bytes.Bytes;
import net.consensys.cava.units.bigints.UInt256;

We have different types in Artemis.

[atoulme]

What types do you folks need? Bytes is just an array of bytes with better methods.

FWIW Harmony is using Cava SSZ: https://github.com/harmony-dev/beacon-chain-java/blob/master/ssz/src/main/java/net/consensys/cava/ssz/BytesSSZReaderProxy.java

[atoulme]

This PR should help: #207

[atoulme]

PR #208 brings SSZ to the project.

[jrhea]

resolved by #208