k_smallest(1) and variants: use min[_by]

Like with `tail(1)`, I think we should leverage a possible faster specialization.

src/k_smallest.rs

@@ -45,6 +45,9 @@ where
         iter.last();
         return Vec::new();
     }
+    if k == 1 {
+        return iter.min_by(comparator).into_iter().collect();
+    }
     let mut iter = iter.fuse();
     let mut storage: Vec<I::Item> = iter.by_ref().take(k).collect();
 

src/lib.rs

@@ -2975,6 +2975,9 @@ pub trait Itertools: Iterator {
             self.last();
             return Vec::new().into_iter();
         }
+        if k == 1 {
+            return self.min().into_iter().collect_vec().into_iter();
+        }
 
         let mut iter = self.fuse();
         let mut heap: BinaryHeap<_> = iter.by_ref().take(k).collect();