util:🔑 Provide to_bytes() methods for key types

These are mainly utility methods around the existing way to serialize
the key types.

src/util/key.rs

@@ -44,6 +44,13 @@ impl PublicKey {
         debug_assert!(write_res.is_ok());
     }
 
+    /// Serialize the public key to bytes
+    pub fn to_bytes(&self) -> Vec<u8> {
+        let mut buf = Vec::new();
+        self.write_into(&mut buf);
+        buf
+    }
+
     /// Deserialize a public key from a slice
     pub fn from_slice(data: &[u8]) -> Result<PublicKey, encode::Error> {
         let compressed: bool = match data.len() {
@@ -110,6 +117,11 @@ impl PrivateKey {
         }
     }
 
+    /// Serialize the private key to bytes
+    pub fn to_bytes(&self) -> Vec<u8> {
+        self.key[..].to_vec()
+    }
+
     /// Format the private key to WIF format.
     pub fn fmt_wif(&self, fmt: &mut fmt::Write) -> fmt::Result {
         let mut ret = [0; 34];