Put validator balances into a separate list in the state

[vbuterin]

No description provided.

[vbuterin]

One aesthetic change I think we should consider is switching the get_effective_balance function to just take the state as an argument instead of (as in this PR right now) the validator balance entry. I could see future changes that use parts of the validator registry entry as well as the balance entry to calculate the effective balance, so we may as well just make grab all the data it needs from the state.

[djrtwo]

It might be worth introducing two functions increase_balance(state, validator_index, amount) and decrease_balance(state, validator_index, amount) to mask the changes in validator balance so much of the code doesn't have to think about dealing with validator_balances separate from `validator_registry.

[djrtwo]

@vbuterin I'm going to clean up per my recommendations and see if we like it.

[vbuterin]

I updated get_effective_balance to take state and index and fixed a couple other issues.

[djrtwo]

A few small bugs that you can fix with a click.

Other than that, approved!