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/aggregate_public_key.rs

@@ -0,0 +1,24 @@
+use super::PublicKey;
+use bls_aggregates::AggregatePublicKey as RawAggregatePublicKey;
+
+/// A single BLS signature.
+///
+/// This struct is a wrapper upon a base type and provides helper functions (e.g., SSZ
+/// serialization).
+#[derive(Debug, Clone)]
+pub struct AggregatePublicKey(RawAggregatePublicKey);
+
+impl AggregatePublicKey {
+    pub fn new() -> Self {
+        AggregatePublicKey(RawAggregatePublicKey::new())
+    }
+
+    pub fn add(&mut self, public_key: &PublicKey) {
+        self.0.add(public_key.as_raw())
+    }
+
+    /// Returns the underlying signature.
+    pub fn as_raw(&self) -> &RawAggregatePublicKey {
+        &self.0
+    }
+}