[Mega Tracking] Align Codebase to Spec v0.3

[terencechain]

This issue tracks the overall progress of updating the current beacon chain implementation to spec v0.3 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
tighten up eth1 deposit flow	594 589
enable transfers btw "withdrawable" accounts	601
switch bitfields to little-endian	575	#1628
only reward attestations from committees for previous epoch + note on validator set to reward/penalize	597
add minimum slashing penalty	624
refactor withdrawable	615

🐛 Bugs:

Descriptions	Eth2.0 spec PR	Prysm PR
fix off by one error in attestation validity	627	#1619
fix block slot acceptable processing time	636	#1611
minor bugs related to ordering/typos/var-mismatch/etc	619 635 637	#1620

🐘 Misc:

Descriptions	Eth2.0 spec PR	Prysm PR
clarifying note on unix time and leap seconds	600	#1622
safety assert on get_permuted_index	622
variable name changes for clarity	534	#1621
minor formatting/typos/etc PRs	560 607 608 614	#1611

Phase 0 validator spec

Descriptions	Eth2.0 spec PR	Prysm PR
variable name changes	630	#1733
minor bugfix when getting epoch_boundary_root	629	#1733

Simple Serialize

Descriptions	Eth2.0 spec PR	Prysm PR
simplify bytes type	596
fix merkleization 0-chunk bug	602
merkleize container elements	595

[terencechain]

Closed for #2307