Validate bytecode at a given contract address

[ethanfrey]

Builds on #810

Given an address on the ethereum chain, we should be able to get the bytecode loaded there (or no bytecode).

Method like getContractAtAddress()

• [ ] Expose reading the bytecode at an address as a method on EthereumConnection
• [ ] Validate the contents of a previously uploaded contract (as a test)
• [ ] Verify if this is deterministic (solidity -> bytecode), always? for one version of solc? never?
• [ ] Bonus: check if it is possible to reverse compile the bytecode

[ethanfrey]

I think this does not belong in iov-core itself, but rather tooling (like we have for deployment), so we can make use of tons of libraries without adding all those dependencies.

Still, a useful tool to validate if a local solidity contract matches some deployed bytecode