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/network/src/service.rs

@@ -5,7 +5,7 @@ use crate::{
     NetworkConfig,
 };
 use crate::{error, metrics};
-use beacon_chain::{BeaconChain, BeaconChainTypes};
+use beacon_chain::{BeaconChain, BeaconChainError, BeaconChainTypes};
 use eth2_libp2p::{
     rpc::{GoodbyeReason, RPCResponseErrorCode, RequestId},
     Gossipsub, Libp2pEvent, PeerAction, PeerRequestId, PubsubMessage, Request, Response,
@@ -281,7 +281,7 @@ fn spawn_service<T: BeaconChainTypes>(
                 _ = service.gossipsub_parameter_update.next() => {
                     if let Ok(slot) = service.beacon_chain.slot() {
                         if let Some(active_validators) = service.beacon_chain.with_head(|head| {
-                                Ok(
+                                Ok::<_, BeaconChainError>(
                                     head
                                     .beacon_state
                                     .get_cached_active_validator_indices(RelativeEpoch::Current)