Add clear function that clears the linked list.

src/ll.c

@@ -113,3 +113,14 @@ void* ll_poptail(ll* ll) {
   }
   return to_return;
 }
+
+void ll_clear(ll* ll){
+  ll_node* head = ll->head;
+  while(ll->head){
+    ll_node* to_free = head;
+    head = head->next;
+    free(to_free);
+  }
+  ll->head = NULL;
+  ll->tail = NULL;
+}