state-migration: Handle accounts with existing balance

[palango]

Fixes https://github.com/celo-org/optimism/issues/158

Improves the checks during copying of the allocs into the state:

• Add any existing balance to the copied account. This happens when the account already has (native) balance in the state.
• Only check contract code when the current code is not empty. This fixes some misleading warnings.
• Add a whitelist for contracts to overwrite. Currently it only includes the Create2Deployer. @pahor167 Is that something that you want to check?