feat(ll): provide a ds_*ll_init func for consistency

ds.c

@@ -3,6 +3,18 @@
 
 #include "ds.h"
 
+ds_sll_t
+*ds_sll_init(void)
+{
+	return ds_sll_new_node(NULL);
+}
+
+ds_dll_t
+*ds_dll_init(void)
+{
+	return ds_dll_new_node(NULL);
+}
+
 ds_sll_t
 *ds_sll_new_node(void *data)
 {

ds.h

@@ -24,6 +24,20 @@ typedef struct {
 
 #define ds_ll_foreach(t, ll) for (t *cur = ll; cur; cur = cur->next)
 
+/**
+ * @brief initialize a new singly linked list
+ *
+ * @return the new linked list
+ */
+ds_sll_t *ds_sll_init(void);
+
+/**
+ * @brief initialize a new doubly linked list
+ *
+ * @return the new linked list
+ */
+ds_dll_t *ds_dll_init(void);
+
 /**
  * @brief create a new allocated node for a singly linked list
  *