feat(StdInvariant): add standard Handler contract for invariant testing

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

```solidity
// 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