Make k_smallest work with just alloc

Author: nbraud

src/k_smallest.rs

@@ -1,5 +1,5 @@
-use std::collections::BinaryHeap;
-use std::cmp::Ord;
+use alloc::collections::BinaryHeap;
+use core::cmp::Ord;
 
 pub(crate) fn k_smallest<T: Ord, I: Iterator<Item = T>>(mut iter: I, k: usize) -> BinaryHeap<T> {
     if k == 0 { return BinaryHeap::new(); }