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/dleq/src/cross_group/mod.rs

@@ -30,14 +30,11 @@ pub(crate) mod aos;
 mod bits;
 use bits::{BitSignature, Bits};
 
-// Feature gated due to MSRV requirements
-#[cfg(feature = "black_box")]
-pub(crate) fn black_box<T>(val: T) -> T {
-  core::hint::black_box(val)
-}
-
-#[cfg(not(feature = "black_box"))]
-pub(crate) fn black_box<T>(val: T) -> T {
+// Use black_box when possible
+#[rustversion::since(1.66)]
+use core::hint::black_box;
+#[rustversion::before(1.66)]
+fn black_box<T>(val: T) -> T {
   val
 }