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

common/directory/src/lib.rs

@@ -1,6 +1,6 @@
 use clap::ArgMatches;
 pub use eth2_testnet_config::DEFAULT_HARDCODED_TESTNET;
-use std::fs::create_dir_all;
+use std::fs::{self, create_dir_all};
 use std::path::{Path, PathBuf};
 
 /// Names for the default directories.
@@ -58,3 +58,21 @@ pub fn parse_path_or_default_with_flag(
             .join(flag),
     )
 }
+
+/// Get the approximate size of a directory and its contents.
+///
+/// Will skip unreadable files, and files. Not 100% accurate if files are being created and deleted
+/// while this function is running.
+pub fn size_of_dir(path: &Path) -> u64 {
+    if let Ok(iter) = fs::read_dir(path) {
+        iter.filter_map(std::result::Result::ok)
+            .map(size_of_dir_entry)
+            .sum()
+    } else {
+        0
+    }
+}
+
+fn size_of_dir_entry(dir: fs::DirEntry) -> u64 {
+    dir.metadata().map(|m| m.len()).unwrap_or(0)
+}