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/client/src/config.rs

@@ -63,6 +63,7 @@ pub struct Config {
     pub eth1: eth1::Config,
     pub http_api: http_api::Config,
     pub http_metrics: http_metrics::Config,
+    pub slasher: Option<slasher::Config>,
 }
 
 impl Default for Config {
@@ -84,6 +85,7 @@ impl Default for Config {
             graffiti: Graffiti::default(),
             http_api: <_>::default(),
             http_metrics: <_>::default(),
+            slasher: None,
         }
     }
 }