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/multiexp/Cargo.toml

@@ -13,6 +13,8 @@ all-features = true
 rustdoc-args = ["--cfg", "docsrs"]
 
 [dependencies]
+rustversion = "1"
+
 zeroize = { version = "^1.5", features = ["zeroize_derive"] }
 
 ff = "0.13"
@@ -27,5 +29,4 @@ k256 = { version = "0.13", features = ["bits"] }
 dalek-ff-group = { path = "../dalek-ff-group" }
 
 [features]
-black_box = []
 batch = ["rand_core"]