FILE 0-print_list.c is a function that prints all the elements of a list_t list.
FILE 1-list_len.c is a function that returns the number of elements in a linked list_t list.
FILE 2-add_node.c is a function that adds a new node at the beginning of a list_t list.
FILE 3-add_node_end.c is a function that adds a new node at the end of a list_t list.
FILE 4-free_list.c is a function that frees a list_t list.