Resolve "Add destroy function"

Closes #9 (closed)

stack.c

@@ -11,6 +11,13 @@ void stack_init(stack *s)
     s->data = malloc(sizeof(int) * DEFAULT_CAPACITY);
 }
 
+void stack_destroy(stack *s){
+    free(s->data);
+    s->data = NULL;
+    s->capacity = -1;
+    s->top = -1;
+}
+
 void stack_peek(stack s, int *value){
     if (!stack_is_empty(s)) {
         *value = s.data[s.top];