Remove magic number

In array initialisation we use magic number 64, this is the secret bytes
length multiplied by 2.

Please note; we still use the magic number 32, left as such because it
is used in various ways and its not immediately clear that using a
single const would be any more descriptive.

Use `SECRET_KEY_SIZE * 2` instead of magic number 64.

src/key.rs

@@ -299,7 +299,7 @@ impl SecretKey {
 impl ::serde::Serialize for SecretKey {
     fn serialize<S: ::serde::Serializer>(&self, s: S) -> Result<S::Ok, S::Error> {
         if s.is_human_readable() {
-            let mut buf = [0u8; 64];
+            let mut buf = [0u8; constants::SECRET_KEY_SIZE * 2];
             s.serialize_str(::to_hex(&self.0, &mut buf).expect("fixed-size hex serialization"))
         } else {
             s.serialize_bytes(&self[..])
@@ -925,7 +925,7 @@ impl str::FromStr for KeyPair {
 impl ::serde::Serialize for KeyPair {
     fn serialize<S: ::serde::Serializer>(&self, s: S) -> Result<S::Ok, S::Error> {
         if s.is_human_readable() {
-            let mut buf = [0u8; 64];
+            let mut buf = [0u8; constants::SECRET_KEY_SIZE * 2];
             s.serialize_str(::to_hex(&self.serialize_secret(), &mut buf)
                 .expect("fixed-size hex serialization"))
         } else {

src/secret.rs

@@ -91,7 +91,7 @@ pub struct DisplaySecret {
 impl fmt::Debug for DisplaySecret {
     #[inline]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let mut slice = [0u8; 64];
+        let mut slice = [0u8; SECRET_KEY_SIZE * 2];
         let hex = to_hex(&self.secret, &mut slice).expect("fixed-size hex serializer failed");
         f.debug_tuple("DisplaySecret")
             .field(&hex)