Pass failed gossip blocks to the slasher (#2047)

## Issue Addressed

Closes #2042

## Proposed Changes

Pass blocks that fail gossip verification to the slasher. Blocks that are successfully verified are not passed immediately, but will be passed as part of full block verification.

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -235,7 +235,7 @@ pub struct BeaconChain<T: BeaconChainTypes> {
     /// Arbitrary bytes included in the blocks.
     pub(crate) graffiti: Graffiti,
     /// Optional slasher.
-    pub(crate) slasher: Option<Arc<Slasher<T::EthSpec>>>,
+    pub slasher: Option<Arc<Slasher<T::EthSpec>>>,
 }
 
 type BeaconBlockAndState<T> = (BeaconBlock<T>, BeaconState<T>);