Add agg_pub to bls, add agg_sig.verify_multiple

- Adds a new-type wrapper for `AggregatePublicKey`, just like all the
other types.
- Adds the `verify_multiple` method to the `AggregateSignature` newtype,
as was introduced in a recent version of signature-schemes.

eth2/utils/bls/src/lib.rs

@@ -1,20 +1,20 @@
 extern crate bls_aggregates;
 extern crate ssz;
 
+mod aggregate_public_key;
 mod aggregate_signature;
 mod keypair;
 mod public_key;
 mod secret_key;
 mod signature;
 
+pub use crate::aggregate_public_key::AggregatePublicKey;
 pub use crate::aggregate_signature::AggregateSignature;
 pub use crate::keypair::Keypair;
 pub use crate::public_key::PublicKey;
 pub use crate::secret_key::SecretKey;
 pub use crate::signature::Signature;
 
-pub use self::bls_aggregates::AggregatePublicKey;
-
 pub const BLS_AGG_SIG_BYTE_SIZE: usize = 96;
 
 use ssz::ssz_encode;