Implement tree hash caching (#584)

* Implement basic tree hash caching

* Use spaces to indent top-level Cargo.toml

* Optimize BLS tree hash by hashing bytes directly

* Implement tree hash caching for validator registry

* Persist BeaconState tree hash cache to disk

* Address Paul's review comments

eth2/utils/cached_tree_hash/src/lib.rs

@@ -0,0 +1,31 @@
+mod cache;
+mod impls;
+mod multi_cache;
+#[cfg(test)]
+mod test;
+
+pub use crate::cache::TreeHashCache;
+pub use crate::impls::int_log;
+pub use crate::multi_cache::MultiTreeHashCache;
+use ethereum_types::H256 as Hash256;
+use tree_hash::TreeHash;
+
+#[derive(Debug, PartialEq)]
+pub enum Error {
+    /// Attempting to provide more than 2^depth leaves to a Merkle tree is disallowed.
+    TooManyLeaves,
+    /// Shrinking a Merkle tree cache by providing it with less leaves than it currently has is
+    /// disallowed (for simplicity).
+    CannotShrink,
+    /// Cache is inconsistent with the list of dirty indices provided.
+    CacheInconsistent,
+}
+
+/// Trait for types which can make use of a cache to accelerate calculation of their tree hash root.
+pub trait CachedTreeHash<Cache>: TreeHash {
+    /// Create a new cache appropriate for use with values of this type.
+    fn new_tree_hash_cache() -> Cache;
+
+    /// Update the cache and use it to compute the tree hash root for `self`.
+    fn recalculate_tree_hash_root(&self, cache: &mut Cache) -> Result<Hash256, Error>;
+}