Specialize count, last and nth PutBack iterator for performance

They were missing and did hurt the MergeJoinBy performance improvement on the `nth` method because we couldn't borrow iter without exposing new interfaces or using something like the `take_mut` crate.

src/adaptors/mod.rs

@@ -243,6 +243,26 @@ impl<I> Iterator for PutBack<I>
         size_hint::add_scalar(self.iter.size_hint(), self.top.is_some() as usize)
     }
 
+    fn count(self) -> usize {
+        self.iter.count() + (self.top.is_some() as usize)
+    }
+
+    fn last(self) -> Option<Self::Item> {
+        self.iter.last().or(self.top)
+    }
+
+    fn nth(&mut self, n: usize) -> Option<Self::Item> {
+        if let Some(top) = self.top.take() {
+            if n == 0 {
+                Some(top)
+            } else {
+                self.iter.nth(n - 1)
+            }
+        } else {
+            self.iter.nth(n)
+        }
+    }
+
     fn all<G>(&mut self, mut f: G) -> bool
         where G: FnMut(Self::Item) -> bool
     {