Set Graffiti via CLI (#1320)

## Issue Addressed

Closes #1319 

## Proposed Changes

This issue:
1. Allows users to edit their Graffiti via the cli option `--graffiti`. If the graffiti is too long, lighthouse will not start and throw an error message. Otherwise, it will set the Graffiti to be the one provided by the user, right-padded with 0s.
2. Create a new `Graffiti` type and unify the code around it. With this type, everything is enforced at compile-time, and the code can be (I think...) panic-free! :)

## Additional info

Currently, only `&str` are supported, as this is the returned type by `.arg("graffiti")`.
Since this is user-input, I tried being as careful as I could. This is also why I created the `Graffiti` type, to make sure I could check as much as possible at compile time.

consensus/types/src/beacon_block.rs

@@ -41,7 +41,7 @@ impl<T: EthSpec> BeaconBlock<T> {
                     block_hash: Hash256::zero(),
                     deposit_count: 0,
                 },
-                graffiti: [0; 32],
+                graffiti: Graffiti::default(),
                 proposer_slashings: VariableList::empty(),
                 attester_slashings: VariableList::empty(),
                 attestations: VariableList::empty(),

consensus/types/src/beacon_block_body.rs

@@ -1,5 +1,5 @@
 use crate::test_utils::TestRandom;
-use crate::utils::{graffiti_from_hex_str, graffiti_to_hex_str};
+use crate::utils::{graffiti_from_hex_str, graffiti_to_hex_str, Graffiti};
 use crate::*;
 
 use serde_derive::{Deserialize, Serialize};
@@ -21,7 +21,7 @@ pub struct BeaconBlockBody<T: EthSpec> {
         serialize_with = "graffiti_to_hex_str",
         deserialize_with = "graffiti_from_hex_str"
     )]
-    pub graffiti: [u8; 32],
+    pub graffiti: Graffiti,
     pub proposer_slashings: VariableList<ProposerSlashing, T::MaxProposerSlashings>,
     pub attester_slashings: VariableList<AttesterSlashing<T>, T::MaxAttesterSlashings>,
     pub attestations: VariableList<Attestation<T>, T::MaxAttestations>,

consensus/types/src/lib.rs

@@ -100,3 +100,4 @@ pub use bls::{
     Signature, SignatureBytes,
 };
 pub use ssz_types::{typenum, typenum::Unsigned, BitList, BitVector, FixedVector, VariableList};
+pub use utils::{Graffiti, GRAFFITI_BYTES_LEN};

consensus/types/src/utils/serde_utils.rs

@@ -4,6 +4,11 @@ use serde::{Deserialize, Deserializer, Serializer};
 pub const FORK_BYTES_LEN: usize = 4;
 pub const GRAFFITI_BYTES_LEN: usize = 32;
 
+/// Type for a slice of `GRAFFITI_BYTES_LEN` bytes.
+///
+/// Gets included inside each `BeaconBlockBody`.
+pub type Graffiti = [u8; GRAFFITI_BYTES_LEN];
+
 pub fn u8_from_hex_str<'de, D>(deserializer: D) -> Result<u8, D::Error>
 where
     D: Deserializer<'de>,
@@ -92,10 +97,7 @@ where
     serializer.serialize_str(&hex_string)
 }
 
-pub fn graffiti_to_hex_str<S>(
-    bytes: &[u8; GRAFFITI_BYTES_LEN],
-    serializer: S,
-) -> Result<S::Ok, S::Error>
+pub fn graffiti_to_hex_str<S>(bytes: &Graffiti, serializer: S) -> Result<S::Ok, S::Error>
 where
     S: Serializer,
 {
@@ -105,12 +107,12 @@ where
     serializer.serialize_str(&hex_string)
 }
 
-pub fn graffiti_from_hex_str<'de, D>(deserializer: D) -> Result<[u8; GRAFFITI_BYTES_LEN], D::Error>
+pub fn graffiti_from_hex_str<'de, D>(deserializer: D) -> Result<Graffiti, D::Error>
 where
     D: Deserializer<'de>,
 {
     let s: String = Deserialize::deserialize(deserializer)?;
-    let mut array = [0 as u8; GRAFFITI_BYTES_LEN];
+    let mut array = Graffiti::default();
 
     let start = s
         .as_str()