Resolve while_let_on_iterator clippy lint

    error: this loop could be written as a `for` loop
       --> src/parse.rs:464:5
        |
    464 |     while let Some((i, ch)) = chars.next() {
        |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `for (i, ch) in &mut chars`
        |
        = note: `-D clippy::while-let-on-iterator` implied by `-D clippy::all`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#while_let_on_iterator

Author: dtolnay

src/parse.rs

@@ -461,7 +461,7 @@ fn cooked_byte_string(mut input: Cursor) -> Result<Cursor, Reject> {
 fn raw_string(input: Cursor) -> Result<Cursor, Reject> {
     let mut chars = input.char_indices();
     let mut n = 0;
-    while let Some((i, ch)) = chars.next() {
+    for (i, ch) in &mut chars {
         match ch {
             '"' => {
                 n = i;