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/int_to_bytes/src/lib.rs

@@ -53,6 +53,14 @@ pub fn int_to_bytes32(int: u64) -> Vec<u8> {
     bytes.to_vec()
 }
 
+/// Returns `int` as little-endian bytes with a length of 32.
+pub fn int_to_fixed_bytes32(int: u64) -> [u8; 32] {
+    let mut bytes = [0; 32];
+    let int_bytes = int.to_le_bytes();
+    bytes[0..int_bytes.len()].copy_from_slice(&int_bytes);
+    bytes
+}
+
 /// Returns `int` as little-endian bytes with a length of 48.
 pub fn int_to_bytes48(int: u64) -> Vec<u8> {
     let mut bytes = BytesMut::with_capacity(48);
@@ -76,6 +84,13 @@ mod tests {
     use std::{fs::File, io::prelude::*, path::PathBuf};
     use yaml_rust::yaml;
 
+    #[test]
+    fn fixed_bytes32() {
+        for x in &[0, 1, 3, 256, 1024, 2943784] {
+            assert_eq!(&int_to_bytes32(*x), &int_to_fixed_bytes32(*x));
+        }
+    }
+
     #[test]
     fn int_to_bytes3_returns_none() {
         assert_eq!(int_to_bytes3(2_u32.pow(24)), None);