Use SignatureBytes and PublicKeyBytes for deposits (#472)

* Replace deposit signatures with SignatureBytes, a struct which lazyly parsers signatures only on demand.

* check byte length when parsing SignatureBytes

* add comment to struct

* distinguish BadSignature and BadSignatureBytes in verify_deposit_signature

* add test for valid signature

* Implements TryInto<Signature> for &SignatureBytes and From<Signature> for &SignatureBytes

* add and use PublicKeyBytes + fix formatting

* fix compiler warning + docs for macro generated structs

* adds tests to ensure correct byte lengths

* small style improvement as suggested by michaelsproul

eth2/utils/bls/src/signature.rs

@@ -155,6 +155,15 @@ mod tests {
         assert_eq!(original, decoded);
     }
 
+    #[test]
+    pub fn test_byte_size() {
+        let keypair = Keypair::random();
+
+        let signature = Signature::new(&[42, 42], 0, &keypair.sk);
+        let bytes = ssz_encode(&signature);
+        assert_eq!(bytes.len(), BLS_SIG_BYTE_SIZE);
+    }
+
     #[test]
     // TODO: once `CachedTreeHash` is fixed, this test should _not_ panic.
     #[should_panic]