* reorder ssz
* split into hash_trees and ssz_serialization, roughly, for hashing and
IO
* move bitseqs into ssz (from stew)
* clean up imports
* docs, imports
* in makeBeaconBlock - use rollback instead
* in tests - this helps state_sim give more accurate data and makes it
30% faster
* fix some usages of raw BeaconState
When replaying state transitions, for the slots that have a block, the
state root is taken from the block. For slots that lack a block, it's
currently calculated using hash_tree_root which is expensive.
Caching the empty slot state roots helps us avoid recalculating this
hash, meaning that for replay, hashes are never calculated. This turns
blocks into fairly lightweight "state-diffs"!
* avoid re-saving state when replaying blocks
* advance empty slots slot-by-slot and save root
* fix sim randomness
* fix sim genesis filename
* introduce `isEpoch` to check if a slot is an epoch slot
Jacek Sieka