Add ability to retrieve private keys as bytes

This feature is needed for compatibility with LDKLite, where the
initial entropy given to LDK is the private key of the root of
the BIP32 derivation tree.

Closes #188

src/bdk.udl

@@ -283,6 +283,8 @@ interface DescriptorSecretKey {
 
   DescriptorPublicKey as_public();
 
+  sequence<u8> secret_key_bytes();
+
   string as_string();
 };
 

src/lib.rs

@@ -907,6 +907,22 @@ impl DescriptorSecretKey {
         })
     }
 
+    /// Get the private key as bytes.
+    fn secret_key_bytes(&self) -> Vec<u8> {
+        let descriptor_secret_key = self.descriptor_secret_key_mutex.lock().unwrap();
+        let secret_key_bytes: Vec<u8> = match descriptor_secret_key.deref() {
+            BdkDescriptorSecretKey::XPrv(descriptor_x_key) => {
+                descriptor_x_key.xkey.private_key.secret_bytes().to_vec()
+            }
+            BdkDescriptorSecretKey::SinglePriv(descriptor_x_key) => {
+                // unreachable!()
+                descriptor_x_key.key.inner.secret_bytes().to_vec()
+            }
+        };
+
+        secret_key_bytes
+    }
+
     fn as_string(&self) -> String {
         self.descriptor_secret_key_mutex.lock().unwrap().to_string()
     }