Avoid Printing Binary String to Logs (#1576)

Converts the graffiti binary data to string before printing to logs.

## Issue Addressed

#1566 

## Proposed Changes
Rather than converting graffiti to a vector the binary data less the last character is passed to String::from_utf_lossy(). This then allows us to call the to_string() function directly to give us the string

## Additional Info

Rust skills are fairly weak

Cargo.lock

@@ -348,6 +348,7 @@ dependencies = [
  "rand 0.7.3",
  "rand_core 0.5.1",
  "rayon",
+ "regex",
  "safe_arith",
  "serde",
  "serde_derive",

beacon_node/beacon_chain/Cargo.toml

@@ -58,3 +58,4 @@ environment = { path = "../../lighthouse/environment" }
 bus = "2.2.3"
 derivative = "2.1.1"
 itertools = "0.9.0"
+regex = "1.3.9"

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -31,6 +31,7 @@ use fork_choice::ForkChoice;
 use itertools::process_results;
 use operation_pool::{OperationPool, PersistedOperationPool};
 use parking_lot::RwLock;
+use regex::bytes::Regex;
 use slog::{crit, debug, error, info, trace, warn, Logger};
 use slot_clock::SlotClock;
 use state_processing::{
@@ -1319,8 +1320,11 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
         block: SignedBeaconBlock<T::EthSpec>,
     ) -> Result<GossipVerifiedBlock<T>, BlockError<T::EthSpec>> {
         let slot = block.message.slot;
-        let graffiti_string = String::from_utf8(block.message.body.graffiti[..].to_vec())
-            .unwrap_or_else(|_| format!("{:?}", &block.message.body.graffiti[..]));
+        #[allow(clippy::invalid_regex)]
+        let re = Regex::new("\\p{C}").expect("regex is valid");
+        let graffiti_string =
+            String::from_utf8_lossy(&re.replace_all(&block.message.body.graffiti[..], &b""[..]))
+                .to_string();
 
         match GossipVerifiedBlock::new(block, self) {
             Ok(verified) => {