Make Combinations lazy

To make the `Combinations` adaptor lazy, the operation "prefill the lazy buffer" is now done when handling the first element rather than at definition.

However, the value returned by the `n` method is changed when called on a fresh `it.combinations(k)` since the "prefill" operation was delayed.
I'm perplex on why the method `n` was made public in the first place as the value is subject to change (said in its documentation). But it is public and this change is therefore a breaking one.

src/combinations.rs

@@ -37,12 +37,9 @@ pub fn combinations<I>(iter: I, k: usize) -> Combinations<I>
 where
     I: Iterator,
 {
-    let mut pool = LazyBuffer::new(iter);
-    pool.prefill(k);
-
     Combinations {
         indices: (0..k).collect(),
-        pool,
+        pool: LazyBuffer::new(iter),
         first: true,
     }
 }
@@ -107,6 +104,7 @@ where
     type Item = Vec<I::Item>;
     fn next(&mut self) -> Option<Self::Item> {
         if self.first {
+            self.pool.prefill(self.k());
             if self.k() > self.n() {
                 return None;
             }

tests/test_std.rs

@@ -1044,7 +1044,7 @@ fn combinations_inexact_size_hints() {
         let len = binomial(real_n, k);
         assert_eq!(len, it.clone().count());
 
-        let mut nb_loaded = numbers.by_ref().take(k).count(); // because of `LazyBuffer::prefill(k)`
+        let mut nb_loaded = 0;
         let sh = numbers.size_hint();
         assert_eq!(binomial(sh.0 + nb_loaded, k), it.size_hint().0);
         assert_eq!(sh.1.map(|n| binomial(n + nb_loaded, k)), it.size_hint().1);
@@ -1053,8 +1053,10 @@ fn combinations_inexact_size_hints() {
             let elem = it.next();
             assert!(elem.is_some());
             assert_eq!(len - next_count, it.clone().count());
-            // It does not load anything more the very first time (it's prefilled).
-            if next_count > 1 {
+            if next_count == 1 {
+                // The very first time, the lazy buffer is prefilled.
+                nb_loaded = numbers.by_ref().take(k).count();
+            } else {
                 // Then it loads one item each time until exhausted.
                 let nb = numbers.next();
                 if nb.is_some() {