Add beacon chain data classes

[hwwhww]

What is wrong?

#930 - part 2 - moving the data classes from beacon_chain repo to Py-EVM.

How can it be fixed

In beacon_chain repo, there some classes:

• active_state.py
• attestation_record.py
• block.py
• crosslink_record.py
• crystallized_state.py
• shard_and_committee.py
• validator_record.py

In the Eth2.0 call yesterday, Alexey pointed out some problems with SimpleSerialize. At this point, I suggest we use RLP for now before it's standardized. @djrtwo any thoughts?

Action:

• Move these classes to eth/beacon/types/
• Move tests to tests/core/beacon/types/

Note: there are some handy cache data in ActiveState, I think we can remove it and add some other Py-EVM style cache mechanism later.