ABI support

[mpapierski]

This wasn't researched yet, but doesn't seem like a rocket science. If we know how to compute it manually we can also know how to implement an API for it.

This is necessary to make guac a robust solution without doing things manually.

Requirements

• Encoder (simplified encoder Simplified ABI encoder #27)
• Decoder

References

• https://solidity.readthedocs.io/en/develop/abi-spec.html#abi-json