Implement the rest of block operations

[hwwhww]

What is wrong?

eth2/beacon/state_machines/forks/serenity/operation_processing.py

• process_proposer_slashings(state, block, self.config)
• process_attester_slashings(state, block, self.config)
  • Note: rename CasperSlashing to AttesterSlashing
• process_deposits(state, block, self.config)
• process_voluntary_exits(state, block, self.config)

How can it be fixed

Fill this in if you know how to fix it.

[hwwhww]

process_deposits(state, block, self.config) is pending on #289 and spec for generating the test vectors of deposit tree.