Implement EIP-55: Mixed-case checksum address encoding

[krnak]

Implementation of EIP-55 is crucial for compatibility with the existing ecosystem.

For example using an address from the Rabby wallet with web3 python module retrieves this message:

web3.exceptions.InvalidAddress: ('web3.py only accepts checksum addresses. The software that gave you this non-checksum address should be considered unsafe, please file it as a bug on their platform. Try using an ENS name instead.