Fix clippy::needless_pass_by_ref_mut warning

```
warning: this argument is a mutable reference, but not used mutably
   --> src/ast.rs:945:30
    |
945 |     fn peek_signature(input: &mut TokenIter) -> bool {
    |                              ^^^^^^^^^^^^^^ help: consider changing to: `&TokenIter`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut
    = note: `#[warn(clippy::needless_pass_by_ref_mut)]` on by default
```

src/ast.rs

@@ -942,7 +942,7 @@ pub(crate) mod parsing {
         }
     }
 
-    fn peek_signature(input: &mut TokenIter) -> bool {
+    fn peek_signature(input: &TokenIter) -> bool {
         let fork = &mut input.clone();
         fork.parse_kw_opt("const");
         fork.parse_kw_opt("async");