use to_hex in serde conversion, using stack allocation

# Conflicts:
#	src/key.rs
#	src/macros.rs

Author: elichai

src/key.rs

@@ -218,7 +218,8 @@ 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() {
-            s.collect_str(self)
+            let mut buf = [0u8; constants::SECRET_KEY_SIZE*2];
+            s.serialize_str(::to_hex(&self.0[..], &mut buf).expect("Should never fail, the buffer is big enough."))
         } else {
             s.serialize_bytes(&self[..])
         }
@@ -701,9 +702,10 @@ mod test {
         let pk = PublicKey::from_secret_key(&s, &sk);
         #[cfg(fuzzing)]
         let pk = PublicKey::from_slice(&[0x02, 0x18, 0x84, 0x57, 0x81, 0xf6, 0x31, 0xc4, 0x8f, 0x1c, 0x97, 0x09, 0xe2, 0x30, 0x92, 0x06, 0x7d, 0x06, 0x83, 0x7f, 0x30, 0xaa, 0x0c, 0xd0, 0x54, 0x4a, 0xc8, 0x87, 0xfe, 0x91, 0xdd, 0xd1, 0x66]).expect("pk");
+        let mut buf = [0u8; constants::SECRET_KEY_SIZE * 2];
 
         assert_eq!(
-            sk.to_string(),
+            to_hex(&sk[..], &mut buf).unwrap(),
             "01010101010101010001020304050607ffff0000ffff00006363636363636363"
         );
         assert_eq!(