Sync with the spec - data structures

[hwwhww]

What is wrong?

1. [Fix Attestation ordering: aggregation_bitfield before data #349] Attestation: fix fields ordering https://github.com/ethereum/eth2.0-specs/blob/dev/specs/core/0_beacon-chain.md#attestation
2. [Rework ProposalSignedData to Proposal and signature refactoring #355] Spec PR Refactor signature handling consensus-specs#633
   1. Rename ProposalSignedData to Proposal
   2. Move ProposerSlashing.proposal_signature_1 and ProposerSlashing.proposal_signature_2 into per Proposal's signature
3. [Add Transfer data structure #353] Add Transfer data structure (cherry-pick from Add test_serialization #342)
4. [Update AttesationData: latest_crosslink_root: Hash32 to latest_crosslink: CrosslinkRecord #351] Update AttesationData: latest_crosslink_root: Hash32 to latest_crosslink: CrosslinkRecord

How can it be fixed

Fill this in if you know how to fix it.