Minor AttestationData cleanup

[JustinDrake]

Suggested refactor of AttestationData for readability:

{
    # LMD GHOST vote
    'slot': Slot,
    'beacon_block_root': Hash,

    # FFG vote
    'source_epoch': Epoch,
    'source_root': Hash,
    'target_root': Hash,

    # Crosslink vote
    'shard': Shard,
    'previous_crosslink': Crosslink,
    'crosslink_data_root': Hash,
}

• Reorder fields and semantically batch by vote type
• Rename justified_epoch to source_epoch and justified_block_root to source_root (more to the point and shorter).
• Rename epoch_boundary_root to target_root (more to the point and shorter)
• Rename latest_crosslink to previous_crosslink (more consistent/correct name)

[JustinDrake]

Done in #756.