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/tests/wrap_around.rs

@@ -0,0 +1,39 @@
+use slasher::{
+    test_utils::{indexed_att, logger},
+    Config, Slasher,
+};
+use tempdir::TempDir;
+use types::Epoch;
+
+#[test]
+fn attestation_pruning_empty_wrap_around() {
+    let tempdir = TempDir::new("slasher").unwrap();
+    let mut config = Config::new(tempdir.path().into());
+    config.validator_chunk_size = 1;
+    config.chunk_size = 16;
+    config.history_length = 16;
+
+    let slasher = Slasher::open(config.clone(), logger()).unwrap();
+
+    let v = vec![0];
+    let history_length = config.history_length as u64;
+
+    let mut current_epoch = Epoch::new(history_length - 1);
+
+    slasher.accept_attestation(indexed_att(v.clone(), 0, history_length - 1, 0));
+    slasher.process_queued(current_epoch).unwrap();
+    slasher.prune_database(current_epoch).unwrap();
+
+    // Delete the previous attestation
+    current_epoch = Epoch::new(2 * history_length + 2);
+    slasher.prune_database(current_epoch).unwrap();
+
+    // Add an attestation that would be surrounded with the modulo considered
+    slasher.accept_attestation(indexed_att(
+        v.clone(),
+        2 * history_length - 3,
+        2 * history_length - 2,
+        1,
+    ));
+    slasher.process_queued(current_epoch).unwrap();
+}