Improvements for `load-state-diff`

With https://github.com/runtimeverification/kontrol/issues/266, we can now initialize the init node's <accounts> cell with data from a JSON file.

We can improve this further by:

• making the missing fields of an account coming from the JSON file symbolic instead of zero or empty. i.e. Initialize the accounts using {'balance': KVariable('ACCT_X_BAL'), 'nonce': KVariable('ACCT_X_NONCE') , ...} instead of {'balance': intToken(0), 'nonce': intToken(0),...}
• insert the storage updates from the summary in <origStorage>, not only in <storage>.
• TBD