Ensure fixed size Account records

[kroezone]

There is a potential exploit with Memory Accounting if the Account records are not fixed size:

1. Execute some setup code such that:
2. Memory accounting causes at least one Account size to increase
3. Execute some more code to make the Account size decrease (e.g. a transfer)
4. Enjoy a memory refund
5. Sell the memory for a profit
6. Repeat

Might not be practical (the transaction of triggering the refund of 1-2 bytes may not be economically viable), but still a risk to plug. BEst solution is probably just to make balance and allowance 64-bit