bls: debugging utils and fake sig fix

There was a discrepancy between the is_empty fields of fake signatures during testing, so I've added
a small hack to set the is_empty field of a fake signature based on the byte content.
Alternatively, we could just make it so that any fake signature is defined to be equal to any other.

eth2/utils/bls/src/fake_aggregate_public_key.rs

@@ -1,5 +1,4 @@
 use super::{PublicKey, BLS_PUBLIC_KEY_BYTE_SIZE};
-use milagro_bls::AggregatePublicKey as RawAggregatePublicKey;
 
 /// A BLS aggregate public key.
 ///

eth2/utils/bls/src/fake_public_key.rs

@@ -1,6 +1,5 @@
 use super::{SecretKey, BLS_PUBLIC_KEY_BYTE_SIZE};
 use cached_tree_hash::cached_tree_hash_ssz_encoding_as_vector;
-use milagro_bls::PublicKey as RawPublicKey;
 use serde::de::{Deserialize, Deserializer};
 use serde::ser::{Serialize, Serializer};
 use serde_hex::{encode as hex_encode, HexVisitor};

eth2/utils/bls/src/fake_signature.rs

@@ -59,9 +59,10 @@ impl FakeSignature {
                 expected: BLS_SIG_BYTE_SIZE,
             })
         } else {
+            let is_empty = bytes.iter().all(|x| *x == 0);
             Ok(Self {
                 bytes: bytes.to_vec(),
-                is_empty: false,
+                is_empty,
             })
         }
     }

eth2/utils/bls/src/signature.rs

@@ -101,6 +101,12 @@ impl Signature {
     pub fn is_empty(&self) -> bool {
         self.is_empty
     }
+
+    /// Display a signature as a hex string of its bytes.
+    #[cfg(test)]
+    pub fn as_hex_string(&self) -> String {
+        hex_encode(self.as_bytes())
+    }
 }
 
 impl_ssz!(Signature, BLS_SIG_BYTE_SIZE, "Signature");