Tree hash cache arena (#836)

* Start adding interop genesis state to lcli

* Use more efficient method to generate genesis state

* Remove duplicate int_to_bytes32

* Add lcli command to change state genesis time

* Add option to allow VC to start with unsynced BN

* Set VC to do parallel key loading

* Don't default to dummy eth1 backend

* Add endpoint to dump operation pool

* Add metrics for op pool

* Remove state clone for slot notifier

* Add mem size approximation for tree hash cache

* Avoid cloning tree hash when getting head

* Avoid cloning tree hash when getting head

* Add working arena-based cached tree hash

* Add another benchmark

* Add pre-allocation for caches

* Make cache nullable

* Fix bugs in cache tree hash

* Add validator tree hash optimization

* Optimize hash_concat

* Make hash32_concat return fixed-len array

* Fix failing API tests

* Add new beacon state cache struct

* Add validator-specific cache

* Separate list and values arenas

* Add parallel validator registry hashing

* Remove MultiTreeHashCache

* Remove cached tree hash macro

* Fix failing tree hash test

* Address Michael's comments

* Add CachedTreeHash impl for ef tests

* Fix messy merge conflict

* Rename cache struct, add comments

* Rename cache struct, add comments

* Remove unnecessary mutability

* Wrap iter in result

* Tidy cached tree hash

* Address Michael comments

* Address more comments

* Use ring::Context

eth2/utils/tree_hash/src/lib.rs

@@ -24,7 +24,7 @@ pub fn mix_in_length(root: &[u8], length: usize) -> Vec<u8> {
     let mut length_bytes = length.to_le_bytes().to_vec();
     length_bytes.resize(BYTES_PER_CHUNK, 0);
 
-    eth2_hashing::hash_concat(root, &length_bytes)
+    eth2_hashing::hash32_concat(root, &length_bytes)[..].to_vec()
 }
 
 #[derive(Debug, PartialEq, Clone)]

eth2/utils/tree_hash/src/merkleize_padded.rs

@@ -1,5 +1,5 @@
 use super::BYTES_PER_CHUNK;
-use eth2_hashing::{hash, hash_concat, ZERO_HASHES, ZERO_HASHES_MAX_INDEX};
+use eth2_hashing::{hash, hash32_concat, ZERO_HASHES, ZERO_HASHES_MAX_INDEX};
 
 /// The size of the cache that stores padding nodes for a given height.
 ///
@@ -138,7 +138,7 @@ pub fn merkleize_padded(bytes: &[u8], min_leaves: usize) -> Vec<u8> {
                 "Both children should be `BYTES_PER_CHUNK` bytes."
             );
 
-            let hash = hash_concat(left, right);
+            let hash = hash32_concat(left, right);
 
             // Store a parent node.
             chunks