Persist eth1 cache (#760)

* Add intermediate structures for bytes conversion

* Expose byte conversion methods from `Eth1Service`

* Add eth1 ssz containers

* Fix type errors

* Load eth1 cache on restart

* Fix compile errors

* Update Cargo.lock

* Add comments and minor formatting

* Add test for eth1 cache persistence

* Restrict Deposit and Block cache field visibility

* Add checks

* Fix `SszDepositCache` check

* Implement Encode/Decode directly on `BlockCache`

beacon_node/eth1/src/deposit_cache.rs

@@ -1,5 +1,6 @@
 use crate::DepositLog;
 use eth2_hashing::hash;
+use ssz_derive::{Decode, Encode};
 use tree_hash::TreeHash;
 use types::{Deposit, Hash256, DEPOSIT_TREE_DEPTH};
 
@@ -79,6 +80,48 @@ impl DepositDataTree {
     }
 }
 
+#[derive(Encode, Decode, Clone)]
+pub struct SszDepositCache {
+    logs: Vec<DepositLog>,
+    leaves: Vec<Hash256>,
+    deposit_contract_deploy_block: u64,
+    deposit_roots: Vec<Hash256>,
+}
+
+impl SszDepositCache {
+    pub fn from_deposit_cache(cache: &DepositCache) -> Self {
+        Self {
+            logs: cache.logs.clone(),
+            leaves: cache.leaves.clone(),
+            deposit_contract_deploy_block: cache.deposit_contract_deploy_block,
+            deposit_roots: cache.deposit_roots.clone(),
+        }
+    }
+
+    pub fn to_deposit_cache(&self) -> Result<DepositCache, String> {
+        let deposit_tree =
+            DepositDataTree::create(&self.leaves, self.leaves.len(), DEPOSIT_TREE_DEPTH);
+        // Check for invalid SszDepositCache conditions
+        if self.leaves.len() != self.logs.len() {
+            return Err("Invalid SszDepositCache: logs and leaves should have equal length".into());
+        }
+        // `deposit_roots` also includes the zero root
+        if self.leaves.len() + 1 != self.deposit_roots.len() {
+            return Err(
+                "Invalid SszDepositCache: deposit_roots length must be only one more than leaves"
+                    .into(),
+            );
+        }
+        Ok(DepositCache {
+            logs: self.logs.clone(),
+            leaves: self.leaves.clone(),
+            deposit_contract_deploy_block: self.deposit_contract_deploy_block,
+            deposit_tree,
+            deposit_roots: self.deposit_roots.clone(),
+        })
+    }
+}
+
 /// Mirrors the merkle tree of deposits in the eth1 deposit contract.
 ///
 /// Provides `Deposit` objects with merkle proofs included.