Include total difficulty in CHTs and hide implementation details from consumers (#4428)

* 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

ethstore/Cargo.toml

@@ -18,7 +18,7 @@ tiny-keccak = "1.0"
 docopt = { version = "0.6", optional = true }
 time = "0.1.34"
 lazy_static = "0.2"
-itertools = "0.4"
+itertools = "0.5"
 parking_lot = "0.3"
 ethcrypto = { path = "../ethcrypto" }
 ethcore-util = { path = "../util" }