Resolve explicit_iter_loop pedantic clippy lint

    error: it is more concise to loop over references to containers instead of using explicit iteration methods
       --> src/expand.rs:177:16
        |
    177 |     for arg in sig.inputs.iter_mut() {
        |                ^^^^^^^^^^^^^^^^^^^^^ help: to write this more concisely, try: `&mut sig.inputs`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#explicit_iter_loop
        = note: `-D clippy::explicit-iter-loop` implied by `-D clippy::pedantic`

Author: dtolnay

src/expand.rs

@@ -174,7 +174,7 @@ fn transform_sig(
     };
 
     let mut lifetimes = CollectLifetimes::new();
-    for arg in sig.inputs.iter_mut() {
+    for arg in &mut sig.inputs {
         match arg {
             FnArg::Receiver(arg) => lifetimes.visit_receiver_mut(arg),
             FnArg::Typed(arg) => lifetimes.visit_type_mut(&mut arg.ty),