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

eth2/state_processing/src/per_block_processing.rs

@@ -144,8 +144,7 @@ pub fn process_block_header<T: EthSpec>(
 ) -> Result<(), BlockOperationError<HeaderInvalid>> {
     verify!(block.slot == state.slot, HeaderInvalid::StateSlotMismatch);
 
-    let expected_previous_block_root =
-        Hash256::from_slice(&state.latest_block_header.tree_hash_root());
+    let expected_previous_block_root = state.latest_block_header.tree_hash_root();
     verify!(
         block.parent_root == expected_previous_block_root,
         HeaderInvalid::ParentBlockRootMismatch {