Add first implementation of optimised LMD ghost.

- Implements the optimised lmd-ghost fork choice rule.
- Removes Attestations struct.
- Removes latest_attestation_targets from beacon_chain.

beacon_node/beacon_chain/src/attestation_targets.rs

@@ -1,22 +0,0 @@
-use std::collections::HashMap;
-use types::Hash256;
-
-pub struct AttestationTargets {
-    map: HashMap<u64, Hash256>,
-}
-
-impl AttestationTargets {
-    pub fn new() -> Self {
-        Self {
-            map: HashMap::new(),
-        }
-    }
-
-    pub fn get(&self, validator_index: u64) -> Option<&Hash256> {
-        self.map.get(&validator_index)
-    }
-
-    pub fn insert(&mut self, validator_index: u64, block_hash: Hash256) -> Option<Hash256> {
-        self.map.insert(validator_index, block_hash)
-    }
-}