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

Cargo.toml

@@ -68,6 +68,8 @@ members = [
     "remote_signer/backend",
     "remote_signer/client",
 
+    "slasher",
+
     "testing/ef_tests",
     "testing/eth1_test_rig",
     "testing/node_test_rig",

beacon_node/Cargo.toml

@@ -43,3 +43,4 @@ clap_utils = { path = "../common/clap_utils" }
 hyper = "0.13.8"
 lighthouse_version = { path = "../common/lighthouse_version" }
 hex = "0.4.2"
+slasher = { path = "../slasher" }

beacon_node/beacon_chain/Cargo.toml

@@ -61,4 +61,5 @@ derivative = "2.1.1"
 itertools = "0.9.0"
 regex = "1.3.9"
 exit-future = "0.2.0"
+slasher = { path = "../../slasher" }
 eth2 = { path = "../../common/eth2" }