Create EthereumAddress interface and class

By having an interface EthereumAddress we can forget about all the type related issues (string vs hex, etc).

If we have an EthereumAddress obj also we could call toChecksumAddress inside of the constructor and have some utility function like toShortString (which could return `0x1234...1234`), etc.

cc: @danfinlay

Start by reviewing the repository's existing Ethereum address representations and the use of toChecksumAddress. Clarify the interface and class boundary, including constructor normalization and the proposed toShortString utility, before identifying affected callers and tests. Done means the design is agreed and the new abstraction consistently replaces the relevant string-versus-hex handling.

Written by the indexing model from the issue text.