Use iterator instead of manual loop

Get more Rusty, use iterator instead of array access. While we are at it
don't explicitly create the slice, that is a hangover from Rust 1.48

Author: tcharding

examples/taproot.rs

@@ -136,8 +136,8 @@ fn hardcoded_xonlypubkeys() -> Vec<XOnlyPublicKey> {
         ],
     ];
     let mut keys: Vec<XOnlyPublicKey> = vec![];
-    for idx in 0..4 {
-        keys.push(XOnlyPublicKey::from_slice(&serialized_keys[idx][..]).unwrap());
+    for key in serialized_keys {
+        keys.push(XOnlyPublicKey::from_slice(&key).unwrap());
     }
     keys
 }