clippy: add Default impl for checksum engine

Author: apoelstra

src/descriptor/checksum.rs

@@ -79,6 +79,12 @@ pub struct Engine {
     clscount: u64,
 }
 
+impl Default for Engine {
+    fn default() -> Engine {
+        Engine::new()
+    }
+}
+
 impl Engine {
     /// Construct an engine with no input
     pub fn new() -> Self {