fix: drop() decrements bucket size

bucket.go

@@ -26,11 +26,12 @@ func (b bucket[K, V]) get(key K) (value V, found bool) {
 	return slot.value, slot.occupied && b.compare(slot.key, key)
 }
 
-func (b bucket[K, V]) drop(key K) (occupied bool) {
+func (b *bucket[K, V]) drop(key K) (occupied bool) {
 	slot := &b.slots[b.location(key)]
 
 	if slot.occupied && b.compare(slot.key, key) {
 		slot.occupied = false
+		b.size--
 		return true
 	}