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/src/error.rs

@@ -0,0 +1,83 @@
+use crate::Config;
+use std::io;
+use types::{Epoch, Hash256};
+
+#[derive(Debug)]
+pub enum Error {
+    DatabaseError(lmdb::Error),
+    DatabaseIOError(io::Error),
+    SszDecodeError(ssz::DecodeError),
+    BincodeError(bincode::Error),
+    ArithError(safe_arith::ArithError),
+    ChunkIndexOutOfBounds(usize),
+    IncompatibleSchemaVersion {
+        database_schema_version: u64,
+        software_schema_version: u64,
+    },
+    ConfigInvalidChunkSize {
+        chunk_size: usize,
+        history_length: usize,
+    },
+    ConfigInvalidZeroParameter {
+        config: Config,
+    },
+    ConfigIncompatible {
+        on_disk_config: Config,
+        config: Config,
+    },
+    DistanceTooLarge,
+    DistanceCalculationOverflow,
+    /// Missing an attester record that we expected to exist.
+    MissingAttesterRecord {
+        validator_index: u64,
+        target_epoch: Epoch,
+    },
+    AttesterRecordCorrupt {
+        length: usize,
+    },
+    AttesterKeyCorrupt {
+        length: usize,
+    },
+    ProposerKeyCorrupt {
+        length: usize,
+    },
+    MissingIndexedAttestation {
+        root: Hash256,
+    },
+    MissingAttesterKey,
+    MissingProposerKey,
+    AttesterRecordInconsistentRoot,
+}
+
+impl From<lmdb::Error> for Error {
+    fn from(e: lmdb::Error) -> Self {
+        match e {
+            lmdb::Error::Other(os_error) => Error::from(io::Error::from_raw_os_error(os_error)),
+            _ => Error::DatabaseError(e),
+        }
+    }
+}
+
+impl From<io::Error> for Error {
+    fn from(e: io::Error) -> Self {
+        Error::DatabaseIOError(e)
+    }
+}
+
+impl From<ssz::DecodeError> for Error {
+    fn from(e: ssz::DecodeError) -> Self {
+        Error::SszDecodeError(e)
+    }
+}
+
+impl From<bincode::Error> for Error {
+    fn from(e: bincode::Error) -> Self {
+        Error::BincodeError(e)
+    }
+}
+
+impl From<safe_arith::ArithError> for Error {
+    fn from(e: safe_arith::ArithError) -> Self {
+        Error::ArithError(e)
+    }
+}