Allow PeekingNext to be used as a trait object.

This change makes `PeekingNext` object-safe and object-ergonomic by
requiring `Self: Sized` on generic methods and implementing
`PeekingNext` transitively over mutable references. This allows
`PeekingNext` to be used as a trait object just like `Iterator`.

src/peeking_take_while.rs

@@ -16,7 +16,18 @@ pub trait PeekingNext : Iterator {
     /// if `accept` returns true, return it as the next element,
     /// else None.
     fn peeking_next<F>(&mut self, accept: F) -> Option<Self::Item>
-        where F: FnOnce(&Self::Item) -> bool;
+        where Self: Sized,
+              F: FnOnce(&Self::Item) -> bool;
+}
+
+impl<'a, I> PeekingNext for &'a mut I
+    where I: PeekingNext,
+{
+    fn peeking_next<F>(&mut self, accept: F) -> Option<Self::Item>
+        where F: FnOnce(&Self::Item) -> bool
+    {
+        (*self).peeking_next(accept)
+    }
 }
 
 impl<I> PeekingNext for Peekable<I>