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/tests/store_tests.rs

@@ -474,7 +474,7 @@ fn check_chain_dump(harness: &TestHarness, expected_len: u64) {
         // Check that the tree hash of the stored state is as expected
         assert_eq!(
             checkpoint.beacon_state_root,
-            Hash256::from_slice(&checkpoint.beacon_state.tree_hash_root()),
+            checkpoint.beacon_state.tree_hash_root(),
             "tree hash of stored state is incorrect"
         );