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

book/src/SUMMARY.md

@@ -30,6 +30,7 @@
     * [Database Configuration](./advanced_database.md)
 	* [Local Testnets](./local-testnets.md)
     * [Advanced Networking](./advanced_networking.md)
+    * [Running a Slasher](./slasher.md)
 * [Contributing](./contributing.md)
 	* [Development Environment](./setup.md)
 * [FAQs](./faq.md)