Forwards block root iterators (#672)

* Implement forwards block root iterators

* Clean up errors and docs

eth2/lmd_ghost/src/lib.rs

@@ -10,7 +10,7 @@ pub type Result<T> = std::result::Result<T, String>;
 
 // Note: the `PartialEq` bound is only required for testing. If it becomes a serious annoyance we
 // can remove it.
-pub trait LmdGhost<S: Store, E: EthSpec>: PartialEq + Send + Sync + Sized {
+pub trait LmdGhost<S: Store<E>, E: EthSpec>: PartialEq + Send + Sync + Sized {
     /// Create a new instance, with the given `store` and `finalized_root`.
     fn new(store: Arc<S>, finalized_block: &BeaconBlock<E>, finalized_root: Hash256) -> Self;