[Mega Tracking] Align Codebase to Spec v0.4

[terencechain]

This issue tracks the overall progress of updating the current beacon chain implementation to spec v0.4 Here is what has been done so far and the missing steps:

Phase 0 Beacon Chain Spec

🍪 Features:

Descriptions	Eth2.0 spec PR	Prysm PR
refactor signature hashing to use new unified ssz method	633
max list size in get_permuted_index	634
reduce genesis slot to 2**32	655
simpler is_power_of_two implementation	674	#1684
specify lexicographic ordering of crosslink_data_root	676
allow transfers from both non-activated and exited validators	678

🐛 Bugs:

Descriptions	Eth2.0 spec PR	Prysm PR
fix out-of-bounds in get_shuffling	641
fix slot range for attestation inclusion	669	#1715
fix previous epoch logic	672	#1721
fix handling of bad proof of possession in deposit	689	N/A
minor bugs related to ordering/typos/var-mismatch/etc	648 650 654 656 664 661	#1723

🐘 Misc:

Descriptions	Eth2.0 spec PR	Prysm PR
validator slashed and initiated_exited to booleans	658
rename shard_block_root -> crosslink_data_root	662	#1743
minor formatting/typos/etc PRs	640 640 643 646 666	N/A

Phase 0 validator spec

Descriptions	Eth2.0 spec PR	Prysm PR
fix epoch start lookup	671	#1674
make getting assignments more generic	673	#1746

Simple Serialize

`

Descriptions	Eth2.0 spec PR	Prysm PR
unified method for signature hashing	625

[terencechain]

Closed for #2307