Speed up compilation by avoiding zeroize_derive

Cargo.toml

@@ -30,7 +30,7 @@ rand_core = { version = "0.6", default-features = false, optional = true }
 serde_crate = { package = "serde", version = "1.0", default-features = false, optional = true }
 serde_bytes = { version = "0.11", default-features = false, optional = true }
 sha2 = { version = "0.10", default-features = false }
-zeroize = { version = "1", default-features = false, features = ["zeroize_derive"] }
+zeroize = { version = "1", default-features = false }
 
 [dev-dependencies]
 hex = "^0.4"

src/secret.rs

@@ -40,10 +40,14 @@ use crate::signature::*;
 ///
 /// Instances of this secret are automatically overwritten with zeroes when they
 /// fall out of scope.
-#[derive(Zeroize)]
-#[zeroize(drop)] // Overwrite secret key material with null bytes when it goes out of scope.
 pub struct SecretKey(pub(crate) [u8; SECRET_KEY_LENGTH]);
 
+impl Drop for SecretKey {
+    fn drop(&mut self) {
+        self.0.zeroize()
+    }
+}
+
 impl Debug for SecretKey {
     fn fmt(&self, f: &mut ::core::fmt::Formatter<'_>) -> ::core::fmt::Result {
         write!(f, "SecretKey: {:?}", &self.0[..])
@@ -235,13 +239,18 @@ impl<'d> Deserialize<'d> for SecretKey {
 // same signature scheme, and which both fail in exactly the same way.  For a
 // better-designed, Schnorr-based signature scheme, see Trevor Perrin's work on
 // "generalised EdDSA" and "VXEdDSA".
-#[derive(Zeroize)]
-#[zeroize(drop)] // Overwrite secret key material with null bytes when it goes out of scope.
 pub struct ExpandedSecretKey {
     pub(crate) key: Scalar,
     pub(crate) nonce: [u8; 32],
 }
 
+impl Drop for ExpandedSecretKey {
+    fn drop(&mut self) {
+        self.key.zeroize();
+        self.nonce.zeroize()
+    }
+}
+
 impl<'a> From<&'a SecretKey> for ExpandedSecretKey {
     /// Construct an `ExpandedSecretKey` from a `SecretKey`.
     ///