Resolve #268 by adding a Zeroize to DigestTranscript which writes a full block

This is a 'better-than-nothing' attempt to invalidate its state.

Also replaces black_box features with usage of the rustversion crate.

crypto/schnorr/src/aggregate.rs

@@ -139,18 +139,12 @@ impl<C: Ciphersuite> SchnorrAggregate<C> {
 
 /// A signature aggregator capable of consuming signatures in order to produce an aggregate.
 #[allow(non_snake_case)]
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Zeroize)]
 pub struct SchnorrAggregator<C: Ciphersuite> {
   digest: DigestTranscript<C::H>,
   sigs: Vec<SchnorrSignature<C>>,
 }
 
-impl<C: Ciphersuite> Zeroize for SchnorrAggregator<C> {
-  fn zeroize(&mut self) {
-    self.sigs.zeroize();
-  }
-}
-
 impl<C: Ciphersuite> SchnorrAggregator<C> {
   /// Create a new aggregator.
   ///