Refactor tree hashing (#861)

* Pre-allocated tree hash caches

* Add SmallVec to tree hash cache

* Avoid allocation for validator.pubkey

* Avoid iterator which seems to be doing heap alloc

* Add more smallvecs

* MOAR SMALLVEC

* Move non-test code to Hash256 tree hash

* Fix byte ordering error

* Add incomplete but working merkle stream impl

* Fix zero hash error

* Add zero hash fn

* Add MerkleStream comments

* Add smallvec, tidy

* Integrate into tree hash derive

* Update ssz_types tree hash

* Don't heap alloc for mix in length

* Add byte-level streaming to MerkleStream

* Avoid recursion in write method

* Update BLS to MerkleStream

* Fix some not-compiling tests

* Remove debug profiling

* Remove code duplication

* Move beacon state tree hash to new hasher

* Fix failing tests

* Update comments

* Add some fast-paths to tree_hash::merkle_root

* Remove unncessary test

* Rename MerkleStream -> MerkleHasher

* Rename new_with_leaf_count -> with_leaves

* Tidy

* Remove NonZeroUsize

* Remove todo

* Update smallvec

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -1705,7 +1705,7 @@ impl<T: BeaconChainTypes> Drop for BeaconChain<T> {
 
 fn write_state<T: EthSpec>(prefix: &str, state: &BeaconState<T>, log: &Logger) {
     if WRITE_BLOCK_PROCESSING_SSZ {
-        let root = Hash256::from_slice(&state.tree_hash_root());
+        let root = state.tree_hash_root();
         let filename = format!("{}_slot_{}_root_{}.ssz", prefix, state.slot, root);
         let mut path = std::env::temp_dir().join("lighthouse");
         let _ = fs::create_dir_all(path.clone());