[Mega Tracking] Align Codebase to Spec v0.2

[terencechain]

This issue tracks the overall progress of updating the current beacon chain implementation to spec v0.2 Here is what has been done so far and the missing steps:

Phase 0 Beacon Chain:

• introduce "swap-or-not" shuffle
• fork choice rule now weighted by balance Update Fork Choice to Use Balance  #1564
• mix epoch into seed within generate_seed rather than in get_shuffling Update RANDAO to Match Spec #1533
• AttestationData includes latest_crosslink rather than just latest_crosslink_root Update to Use Crosslink in AttestationData  #1546
• add get_previous_epoch helper Aligning ETH2.0 Spec - Implemented Slot & Epoch Helpers #1447
• add get_total_balance helper
• int_to_bytesN uses little-endian Update Contract to Use Little Endian And Encoding Across Entire Repo #1536
• minor range fix to verify_bitfield Implement Verify Bitfield #1562
• Use 2**63 for GENESIS_SLOT Update Genesis Slot and Epoch #1537

Phase 0 validator client:

• implement get_next_epoch_committee_assignments Implement get_next_epoch_committee_assignment helper function #1552

Simple Serialize:

• List tree-hash padding consistency fix
• "internal" vs "external" hash roots padding fix

BLS:

• rename message -> message_hash in all bls functions change to message hash #1563

[terencechain]

Working on issue: AttestationData includes latest_crosslink rather than just latest_crosslink_root

[terencechain]

Working on issue: AttestationData includes latest_crosslink rather than just latest_crosslink_root

Done! Please review #1546

[terencechain]

Working on get_next_epoch_committee_assignments

[cyberbono3]

working on introduce "swap-or-not" shuffle

[terencechain]

working on fork choice rule now weighted by balance

[terencechain]

Closed for #2307