Tail: skip the starting part of iterator

If the iterator is exact sized, then `.collect()` finishes the work.
Thanks to scottmcm!

Author: Philippe-Cholet

src/lib.rs

@@ -3164,7 +3164,9 @@ pub trait Itertools: Iterator {
             }
             1 => self.last().into_iter().collect(),
             _ => {
-                let mut iter = self.fuse();
+                // Skip the starting part of iterator if possible.
+                let (low, _) = self.size_hint();
+                let mut iter = self.fuse().skip(low.saturating_sub(n));
                 let mut data: Vec<_> = iter.by_ref().take(n).collect();
                 // Update `data` cyclically.
                 let idx = iter.fold(0, |i, val| {