Resolve while_let_on_iterator clippy lint

error: this loop could be written as a `for` loop
        --> serde/src/private/de.rs:2835:9
         |
    2835 |         while let Some(item) = self.iter.next() {
         |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `for item in &mut self.iter`
         |
    note: the lint level is defined here
        --> serde/src/lib.rs:97:44
         |
    97   | #![cfg_attr(feature = "cargo-clippy", deny(clippy, clippy_pedantic))]
         |                                            ^^^^^^
         = note: `#[deny(clippy::while_let_on_iterator)]` implied by `#[deny(clippy::all)]`
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#while_let_on_iterator

    error: this loop could be written as a `for` loop
        --> serde/src/private/de.rs:2937:9
         |
    2937 |         while let Some(item) = self.iter.next() {
         |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `for item in &mut self.iter`
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#while_let_on_iterator

serde/src/private/de.rs

@@ -2832,7 +2832,7 @@ where
     where
         T: DeserializeSeed<'de>,
     {
-        while let Some(item) = self.iter.next() {
+        for item in &mut self.iter {
             // Items in the vector are nulled out when used by a struct.
             if let Some((ref key, ref content)) = *item {
                 self.pending_content = Some(content);
@@ -2934,7 +2934,7 @@ where
     where
         T: DeserializeSeed<'de>,
     {
-        while let Some(item) = self.iter.next() {
+        for item in &mut self.iter {
             if let Some((ref key, ref content)) = *item {
                 // Do not take(), instead borrow this entry. The internally tagged
                 // enum does its own buffering so we can't tell whether this entry