Add custom Debug impl for RecoverableSignature

Currently when debug printing the `RecoverableSignature` we do so byte
by byte, this means that the output differs depending on the endianess
of the machine. If instead we serialize the signature in compact form
then the output is the same irrespective of the endianess.

With this applied the following two commands now pass:

```
cargo test test_debug_output --features=recovery

```
cross test --target powerpc-unknown-linux-gnu test_debug_output --features=recovery
```

Fixes: #375

secp256k1-sys/src/recovery.rs

@@ -16,13 +16,13 @@
 //! # FFI of the recovery module
 
 use ::types::*;
-use {Context, Signature, NonceFn, PublicKey};
+use ::core::fmt;
+use {Context, Signature, NonceFn, PublicKey, CPtr};
 
 /// Library-internal representation of a Secp256k1 signature + recovery ID
 #[repr(C)]
 pub struct RecoverableSignature([c_uchar; 65]);
 impl_array_newtype!(RecoverableSignature, c_uchar, 65);
-impl_raw_debug!(RecoverableSignature);
 
 impl RecoverableSignature {
     /// Create a new (zeroed) signature usable for the FFI interface
@@ -35,6 +35,30 @@ impl Default for RecoverableSignature {
     }
 }
 
+impl fmt::Debug for RecoverableSignature {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let mut ret = [0u8; 64];
+        let mut recid = 0i32;
+
+        unsafe {
+            let err = secp256k1_ecdsa_recoverable_signature_serialize_compact(
+                super::secp256k1_context_no_precomp,
+                ret.as_mut_c_ptr(),
+                &mut recid,
+                self,
+            );
+            assert!(err == 1);
+        }
+
+        for byte in ret.iter() {
+            write!(f, "{:02x}", byte)?;
+        }
+        write!(f, "{:02x}", recid as u8)?;
+
+        Ok(())
+    }
+}
+
 extern "C" {
     #[cfg_attr(not(rust_secp_no_symbol_renaming), link_name = "rustsecp256k1_v0_4_1_ecdsa_recoverable_signature_parse_compact")]
     pub fn secp256k1_ecdsa_recoverable_signature_parse_compact(cx: *const Context, sig: *mut RecoverableSignature,

src/ecdsa/recovery.rs

@@ -337,7 +337,7 @@ mod tests {
             0xff, 0x20, 0x80, 0xc4, 0xa3, 0x9a, 0xae, 0x06,
             0x8d, 0x12, 0xee, 0xd0, 0x09, 0xb6, 0x8c, 0x89],
             RecoveryId(1)).unwrap();
-        assert_eq!(&format!("{:?}", sig), "RecoverableSignature(98882e09f4ed6dc3659e43fc771e0cafa60b1f926f2b77041f744721adff7366898cb609d0ee128d06ae9aa3c48020ff9f705e02f80e1280a8ade05216971a4c01)");
+        assert_eq!(&format!("{:?}", sig), "RecoverableSignature(6673ffad2147741f04772b6f921f0ba6af0c1e77fc439e65c36dedf4092e88984c1a971652e0ada880120ef8025e709fff2080c4a39aae068d12eed009b68c8901)");
     }
 
     #[test]