refactor: Update some operation to make them lazy by default.

This doesn't change anything in the behavior of the library.

src/Collection.php

@@ -320,7 +320,7 @@ public function count(): int
 
     public function current(int $index = 0)
     {
-        return Current::of()($index)($this->getIterator());
+        return $this->run(Current::of()($index))->getIterator()->current();
     }
 
     public function cycle(): CollectionInterface
@@ -537,7 +537,7 @@ public function jsonSerialize(): array
 
     public function key(int $index = 0)
     {
-        return Key::of()($index)($this->getIterator());
+        return $this->run(Key::of()($index))->getIterator()->current();
     }
 
     public function keys(): CollectionInterface

src/Operation/Current.php

@@ -36,7 +36,7 @@ static function (Iterator $iterator) use ($index) {
                         for ($i = 0; $i < $index; $i++, $iterator->next()) {
                         }
 
-                        return $iterator->current();
+                        return yield $iterator->current();
                     };
             };
     }

src/Operation/FoldLeft.php

@@ -47,7 +47,12 @@ static function (Iterator $iterator) use ($callback, $initial): Generator {
                                 /** @psalm-var Generator<TKey, T> $iterator */
                                 $iterator = Last::of()(Reduction::of()($callback)($initial)($iterator));
 
-                                return yield Key::of()(0)($iterator) => Current::of()(0)($iterator);
+                                /** @psalm-var Generator<int, TKey> $key */
+                                $key = (Key::of()(0)($iterator));
+                                /** @psalm-var Generator<int, T> $current */
+                                $current = (Current::of()(0)($iterator));
+
+                                return yield $key->current() => $current->current();
                             };
                     };
             };

src/Operation/Key.php

@@ -36,7 +36,7 @@ static function (Iterator $iterator) use ($index) {
                         for ($i = 0; $i < $index; $i++, $iterator->next()) {
                         }
 
-                        return $iterator->key();
+                        return yield $iterator->key();
                     };
             };
     }