feat: VectorDiff::apply uses Vector::append on VectorDiff::Append.

Using `Vector::extend` will push back each value from `values`
individually in a loop. However, this patch changes that to use
`Vector::append` which appends all values at once. It's probably more
optimised.

eyeball-im/src/vector.rs

@@ -433,7 +433,7 @@ impl<T: Clone> VectorDiff<T> {
     pub fn apply(self, vec: &mut Vector<T>) {
         match self {
             VectorDiff::Append { values } => {
-                vec.extend(values);
+                vec.append(values);
             }
             VectorDiff::Clear => {
                 vec.clear();