rust: use `peek_mut` from the standard lib now that it's stable

Just a little cleanup of a TODO found along the way.

rust/hg-core/src/config/mod.rs

@@ -510,30 +510,6 b' impl Config {'
         &'a self,
         section: &'a [u8],
     ) -> impl Iterator<Item = (&[u8], &[u8])> + 'a {
-        // TODO: Use `Iterator`’s `.peekable()` when its `peek_mut` is
-        // available:
-        // https://doc.rust-lang.org/nightly/std/iter/struct.Peekable.html#method.peek_mut
-        struct Peekable<I: Iterator> {
-            iter: I,
-            /// Remember a peeked value, even if it was None.
-            peeked: Option<Option<I::Item>>,
-        }
-
-        impl<I: Iterator> Peekable<I> {
-            fn new(iter: I) -> Self {
-                Self { iter, peeked: None }
-            }
-
-            fn next(&mut self) {
-                self.peeked = None
-            }
-
-            fn peek_mut(&mut self) -> Option<&mut I::Item> {
-                let iter = &mut self.iter;
-                self.peeked.get_or_insert_with(|| iter.next()).as_mut()
-            }
-        }
-
         // Deduplicate keys redefined in multiple layers
         let mut keys_already_seen = HashSet::new();
         let mut key_is_new =
@@ -543,12 +519,12 b' impl Config {'
         // This is similar to `flat_map` + `filter_map`, except with a single
         // closure that owns `key_is_new` (and therefore the
         // `keys_already_seen` set):
-        let mut layer_iters = Peekable::new(
-            self.layers
-                .iter()
-                .rev()
-                .map(move |layer| layer.iter_section(section)),
-        );
+        let mut layer_iters = self
+            .layers
+            .iter()
+            .rev()
+            .map(move |layer| layer.iter_section(section))
+            .peekable();
         std::iter::from_fn(move || loop {
             if let Some(pair) = layer_iters.peek_mut()?.find(&mut key_is_new) {
                 return Some(pair);