* don't write blocks that get added to database
* don't write states
* write to folders
* add state dumping feature to `ncli_db` to get any known state from the
database
* 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
Jacek Sieka