Fork choice bug fixes (#449)

* Change reduced tree for adding weightless node

* Add more comments for reduced tree fork choice

* Small refactor on reduced tree for readability

* Move test_harness forking logic into itself

* Add new `AncestorIter` trait to store

* Add unfinished tests to fork choice

* Make `beacon_state.genesis_block_root` public

* Add failing lmd_ghost fork choice tests

* Extend fork_choice tests, create failing test

* Implement Debug for generic ReducedTree

* Add lazy_static to fork choice tests

* Add verify_integrity fn to reduced tree

* Fix bugs in reduced tree

* Ensure all reduced tree tests verify integrity

* Slightly alter reduce tree test params

* Add (failing) reduced tree test

* Fix bug in fork choice

Iter ancestors was not working well with skip slots

* Put maximum depth for common ancestor search

Ensures that we don't search back past the finalized root.

* Add basic finalization tests for reduced tree

* Change fork choice to use beacon_block_root

Previously it was using target_root, which was wrong

* Make ancestor iter return option

* Disable fork choice test when !debug_assertions

* Fix type, removed code fragment

* Tidy some borrow-checker evading

* Lower reduced tree random test iterations

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -77,7 +77,7 @@ pub struct BeaconChain<T: BeaconChainTypes> {
     /// to `per_slot_processing`.
     state: RwLock<BeaconState<T::EthSpec>>,
     /// The root of the genesis block.
-    genesis_block_root: Hash256,
+    pub genesis_block_root: Hash256,
     /// A state-machine that is updated with information from the network and chooses a canonical
     /// head block.
     pub fork_choice: ForkChoice<T>,