Replace custom advance_by with Iterator::nth

Previously, there was a comment stating that we must wait for [`Iterator::advance_by` to be stabilizied](rust-lang/rust#77404), but it makes more sense to use the [existing `Iterator::nth` method](https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.nth) anyway, so I updated all callsites to use it.

Depending on the iterator implementation, this could result in a performance improvement.

src/iter/iter_mut.rs

@@ -31,28 +31,18 @@ impl<'a, T> Iterator for IterMut<'a, T> {
  fn next(&mut self) -> Option<Self::Item> {
  // Get the next index and update all cursors
  let index = self.occupied.next()?;
- let skip = match self.prev_index {
+ let relative_index = match self.prev_index.replace(index) {
  None => 0,
  Some(prev_index) => index - prev_index - 1,
  };
- self.prev_index = Some(index);
- advance_by(&mut self.entries, skip);
 
  // SAFETY: we just confirmed that there was in fact an entry at this index
  self.entries
- .next()
+ .nth(relative_index)
  .map(|t| (index.into(), unsafe { t.assume_init_mut() }))
  }
 }
 
-// TODO: Waiting for `Iterator::advance_by` to be stabilized
-// https://github.com/rust-lang/rust/issues/77404
-fn advance_by(iter: &mut impl Iterator, n: usize) {
- for _ in 0..n {
- iter.next();
- }
-}
-
 #[cfg(test)]
 mod test {
  use super::*;

src/iter/values_mut.rs

@@ -31,23 +31,15 @@ impl<'a, T> Iterator for ValuesMut<'a, T> {
  fn next(&mut self) -> Option<Self::Item> {
  // Get the next index and update all cursors
  let index = self.occupied.next()?;
- let skip = match self.prev_index {
+ let relative_index = match self.prev_index.replace(index) {
  None => 0,
  Some(prev_index) => index - prev_index - 1,
  };
- self.prev_index = Some(index);
- advance_by(&mut self.entries, skip);
 
  // SAFETY: we just confirmed that there was in fact an entry at this index
- self.entries.next().map(|t| unsafe { t.assume_init_mut() })
- }
-}
-
-// TODO: Waiting for `Iterator::advance_by` to be stabilized
-// https://github.com/rust-lang/rust/issues/77404
-fn advance_by(iter: &mut impl Iterator, n: usize) {
- for _ in 0..n {
- iter.next();
+ self.entries
+ .nth(relative_index)
+ .map(|t| unsafe { t.assume_init_mut() })
  }
 }