Resolve clippy append_instead_of_extend lint

    error: use of `extend` instead of `append` for adding the full range of a second vector
        --> src/item.rs:1811:13
         |
    1811 |             attrs.extend(item_attrs.drain(..));
         |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `attrs.append(&mut item_attrs)`
         |
         = note: `-D clippy::append-instead-of-extend` implied by `-D clippy::all`
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#append_instead_of_extend

    error: use of `extend` instead of `append` for adding the full range of a second vector
        --> src/item.rs:2296:13
         |
    2296 |             attrs.extend(item_attrs.drain(..));
         |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `attrs.append(&mut item_attrs)`
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#append_instead_of_extend

    error: use of `extend` instead of `append` for adding the full range of a second vector
        --> src/item.rs:2640:17
         |
    2640 |                 attrs.extend(item_attrs.drain(..));
         |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `attrs.append(&mut item_attrs)`
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#append_instead_of_extend

src/item.rs

@@ -1808,7 +1808,7 @@ pub mod parsing {
                 #[cfg(not(test))]
                 _ => unreachable!(),
             };
-            attrs.extend(item_attrs.drain(..));
+            attrs.append(item_attrs);
             *item_attrs = attrs;
 
             Ok(item)
@@ -2293,7 +2293,7 @@ pub mod parsing {
                 #[cfg(not(test))]
                 _ => unreachable!(),
             };
-            attrs.extend(item_attrs.drain(..));
+            attrs.append(item_attrs);
             *item_attrs = attrs;
             Ok(item)
         }
@@ -2637,7 +2637,7 @@ pub mod parsing {
                     #[cfg(not(test))]
                     _ => unreachable!(),
                 };
-                attrs.extend(item_attrs.drain(..));
+                attrs.append(item_attrs);
                 *item_attrs = attrs;
             }