Partially implement optimised lmd ghost.

- Creates fork-choice trait.
- Corrects comment spelling in beacon block.
- Implements helper functions of optimised lmd ghost.

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -33,7 +33,7 @@ pub enum Error {
 
 #[derive(Debug, PartialEq)]
 pub enum ValidBlock {
-    /// The block was sucessfully processed.
+    /// The block was successfully processed.
     Processed,
 }