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/eth1/src/deposit_cache.rs

@@ -149,7 +149,7 @@ impl DepositCache {
     pub fn insert_log(&mut self, log: DepositLog) -> Result<(), Error> {
         match log.index.cmp(&(self.logs.len() as u64)) {
             Ordering::Equal => {
-                let deposit = Hash256::from_slice(&log.deposit_data.tree_hash_root());
+                let deposit = log.deposit_data.tree_hash_root();
                 self.leaves.push(deposit);
                 self.logs.push(log);
                 self.deposit_tree