Rollup merge of rust-lang#62481 - czipperz:iterator-last-nth-use-for_each, r=scottmcm

Use `fold` in `Iterator::last` default implementation

We already use it in all the other methods.  Consistency + potential perf is a pretty nice win!

Author: Centril

src/libcore/iter/traits/iterator.rs

@@ -263,9 +263,7 @@ pub trait Iterator {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn last(self) -> Option<Self::Item> where Self: Sized {
-        let mut last = None;
-        for x in self { last = Some(x); }
-        last
+        self.fold(None, |_, x| Some(x))
     }
 
     /// Returns the `n`th element of the iterator.