attesation proposer rewards to block processing

[djrtwo]

Handles #906

• move attestation proposer rewards to block processing
• beacon block proposer can only be deterministically checked during the slot in question because balances might change at every block. thus removal of slot param from get_beacon_proposer_index
• extend some helper functionality to be more modular to be used in this context

todo: write some tests

[hwwhww]

- def get_beacon_proposer_index(state: BeaconState,
-                               slot: Slot) -> ValidatorIndex:
+ def get_beacon_proposer_index(state: BeaconState) -> ValidatorIndex:

But it is useful for block proposal!
https://github.com/ethereum/eth2.0-specs/blob/442aa0e7f34012b37e61f19e156246694ac76997/specs/validator/0_beacon-chain-validator.md#block-proposal

[JustinDrake]

But it is useful for block proposal!

It's a fundamental constraint with the current proposer sampling logic.

[JustinDrake]

Removed ~30 lines and made things more readable... but broke the tests 😂