feat: `Handler` contract

[mds1]

Let's flesh out a standard handler contract that people can build off of. References:

• https://book.getfoundry.sh/forge/invariant-testing
• https://mirror.xyz/horsefacts.eth/Jex2YVaO65dda6zEyfM_-DXlXhOWCAoSpOx5PLocYgw
• https://github.com/lucas-manuel/invariant-examples
• https://github.com/maple-labs/maple-core-v2/tree/main/tests/invariants

// src/Handler.sol
contract Handler is CommonBase, StdCheats, StdUtils {
  // TODO Helpers for managing actors/addresses
  // TODO Helpers for call summary data
  // TODO What else?
}

cc @lucas-manuel @horsefacts @gakonst