* CHT builder and prover
* use CHT abstraction in provider
* hide CHT internals from header chain
* fix itertools conflict by updating all to 0.5
* cht proof checker, use it in on_demand
Closes#2213
Omit transaction and receipt roots from abridged block.
No longer use RLP compression.
Make ordered_trie_root generic over an iterator to save an allocation.
Breaks snapshot format backwards compatibility (with other 1.4 snapshots -- it's already been broken with 1.3).
Documentation will need updating
