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

slasher/src/lib.rs

@@ -0,0 +1,66 @@
+#![deny(missing_debug_implementations)]
+
+mod array;
+mod attestation_queue;
+mod attester_record;
+mod block_queue;
+pub mod config;
+mod database;
+mod error;
+mod metrics;
+mod slasher;
+mod slasher_server;
+pub mod test_utils;
+mod utils;
+
+pub use crate::slasher::Slasher;
+pub use attestation_queue::{AttestationBatch, AttestationQueue};
+pub use attester_record::AttesterRecord;
+pub use block_queue::BlockQueue;
+pub use config::Config;
+pub use database::SlasherDB;
+pub use error::Error;
+pub use slasher_server::SlasherServer;
+
+use types::{AttesterSlashing, EthSpec, IndexedAttestation, ProposerSlashing};
+
+#[derive(Debug, PartialEq)]
+pub enum AttesterSlashingStatus<E: EthSpec> {
+    NotSlashable,
+    /// A weird outcome that can occur when we go to lookup an attestation by its target
+    /// epoch for a surround slashing, but find a different attestation -- indicating that
+    /// the validator has already been caught double voting.
+    AlreadyDoubleVoted,
+    DoubleVote(Box<IndexedAttestation<E>>),
+    SurroundsExisting(Box<IndexedAttestation<E>>),
+    SurroundedByExisting(Box<IndexedAttestation<E>>),
+}
+
+#[derive(Debug, PartialEq)]
+pub enum ProposerSlashingStatus {
+    NotSlashable,
+    DoubleVote(Box<ProposerSlashing>),
+}
+
+impl<E: EthSpec> AttesterSlashingStatus<E> {
+    pub fn into_slashing(
+        self,
+        new_attestation: &IndexedAttestation<E>,
+    ) -> Option<AttesterSlashing<E>> {
+        use AttesterSlashingStatus::*;
+
+        // The surrounding attestation must be in `attestation_1` to be valid.
+        match self {
+            NotSlashable => None,
+            AlreadyDoubleVoted => None,
+            DoubleVote(existing) | SurroundedByExisting(existing) => Some(AttesterSlashing {
+                attestation_1: *existing,
+                attestation_2: new_attestation.clone(),
+            }),
+            SurroundsExisting(existing) => Some(AttesterSlashing {
+                attestation_1: new_attestation.clone(),
+                attestation_2: *existing,
+            }),
+        }
+    }
+}