Handle eth transfers as a special case

[Arkenan]

We should not have all the VM work (creating big call frames, caches, etc) if we are processing eth transfers. We should have a simpler VM implementation that just handles eth transfers by changing the balances.

Some references that might be useful:

• To know if a transaction is a transfer, we should check if the input field is present.
• The balance should be updated in both the sender and the receiver.
• The nonce should be updated in the sender.
• The fee should be calculated as TX_BASE_COST and deduced as well.
• Validations should be performed (mostly if the sender has enough balance).

[MegaRedHand]

Note that we also need to verify that the receiver is not a smart contract (i.e. it has an empty code hash). If it has code, we need to execute it.