Implement slasher (#1567)

This is an implementation of a slasher that lives inside the BN and can be enabled via `lighthouse bn --slasher`.

Features included in this PR:

- [x] Detection of attester slashing conditions (double votes, surrounds existing, surrounded by existing)
- [x] Integration into Lighthouse's attestation verification flow
- [x] Detection of proposer slashing conditions
- [x] Extraction of attestations from blocks as they are verified
- [x] Compression of chunks
- [x] Configurable history length
- [x] Pruning of old attestations and blocks
- [x] More tests

Future work:

* Focus on a slice of history separate from the most recent N epochs (e.g. epochs `current - K` to `current - M`)
* Run out-of-process
* Ingest attestations from the chain without a resync

Design notes are here https://hackmd.io/@sproul/HJSEklmPL

beacon_node/beacon_chain/src/builder.rs

@@ -21,6 +21,7 @@ use fork_choice::ForkChoice;
 use futures::channel::mpsc::Sender;
 use operation_pool::{OperationPool, PersistedOperationPool};
 use parking_lot::RwLock;
+use slasher::Slasher;
 use slog::{crit, info, Logger};
 use slot_clock::{SlotClock, TestingSlotClock};
 use std::marker::PhantomData;
@@ -99,6 +100,7 @@ pub struct BeaconChainBuilder<T: BeaconChainTypes> {
     disabled_forks: Vec<String>,
     log: Option<Logger>,
     graffiti: Graffiti,
+    slasher: Option<Arc<Slasher<T::EthSpec>>>,
 }
 
 impl<TSlotClock, TEth1Backend, TEthSpec, TEventHandler, THotStore, TColdStore>
@@ -139,6 +141,7 @@ where
             chain_config: ChainConfig::default(),
             log: None,
             graffiti: Graffiti::default(),
+            slasher: None,
         }
     }
 
@@ -174,6 +177,12 @@ where
         self
     }
 
+    /// Sets the slasher.
+    pub fn slasher(mut self, slasher: Arc<Slasher<TEthSpec>>) -> Self {
+        self.slasher = Some(slasher);
+        self
+    }
+
     /// Sets the logger.
     ///
     /// Should generally be called early in the build chain.
@@ -571,6 +580,7 @@ where
                 .ok_or_else(|| "Cannot build without a shutdown sender.".to_string())?,
             log: log.clone(),
             graffiti: self.graffiti,
+            slasher: self.slasher.clone(),
         };
 
         let head = beacon_chain