Implement PeekingNext for PeekingTakeWhile.

This change implements `PeekingNext` for `PeekingTakeWhile` by composing
its predicate with the predicate given to `PeekingNext::peeking_next`.
This allows subsequent (and chained) calls to functions that require
`PeekingNext` following `Itertools::peeking_take_while`.

src/peeking_take_while.rs

@@ -115,6 +115,18 @@ impl<'a, I, F> Iterator for PeekingTakeWhile<'a, I, F>
     }
 }
 
+impl<'a, I, F> PeekingNext for PeekingTakeWhile<'a, I, F>
+    where I: PeekingNext,
+          F: FnMut(&I::Item) -> bool,
+{
+    fn peeking_next<G>(&mut self, g: G) -> Option<Self::Item>
+        where G: FnOnce(&Self::Item) -> bool,
+    {
+        let f = &mut self.f;
+        self.iter.peeking_next(|r| f(r) && g(r))
+    }
+}
+
 // Some iterators are so lightweight we can simply clone them to save their
 // state and use that for peeking.
 macro_rules! peeking_next_by_clone {

tests/peeking_take_while.rs

@@ -48,3 +48,22 @@ fn peeking_take_while_slice_iter_rev() {
     r.peeking_take_while(|_| true).count();
     assert_eq!(r.next(), None);
 }
+
+#[test]
+fn peeking_take_while_nested() {
+    let mut xs = (0..10).peekable();
+    let ys: Vec<_> = xs
+        .peeking_take_while(|x| *x < 6)
+        .peeking_take_while(|x| *x != 3)
+        .collect();
+    assert_eq!(ys, vec![0, 1, 2]);
+    assert_eq!(xs.next(), Some(3));
+
+    let mut xs = (4..10).peekable();
+    let ys: Vec<_> = xs
+        .peeking_take_while(|x| *x != 3)
+        .peeking_take_while(|x| *x < 6)
+        .collect();
+    assert_eq!(ys, vec![4, 5]);
+    assert_eq!(xs.next(), Some(6));
+}