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/ed448/Cargo.toml

@@ -13,6 +13,8 @@ all-features = true
 rustdoc-args = ["--cfg", "docsrs"]
 
 [dependencies]
+rustversion = "1"
+
 lazy_static = "1"
 
 rand_core = "0.6"
@@ -30,6 +32,3 @@ crypto-bigint = { version = "0.5", features = ["zeroize"] }
 hex = "0.4"
 
 ff-group-tests = { path = "../ff-group-tests" }
-
-[features]
-black_box = []

crypto/ed448/src/backend.rs

@@ -1,13 +1,10 @@
 use zeroize::Zeroize;
 
-// 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
 }