made examples/demos for vectors && files && lists | also fixed a bug with the vector and list size and popping, explained in the adjacent comment

src/list.c

@@ -96,12 +96,17 @@ listCleanup(list_t* l) {
 
 void*
 listPop(list_t* l) {
-    vector_t *first_ptr = (vector_t *)vectorPop((vector_t *)l->data);
-    void *ptr = vectorPop(first_ptr);
-    l->size -= 1;
+    if (l->size != 0) {
+        vector_t *first_ptr = (vector_t *)vectorPop((vector_t *)l->data);
+        void *ptr = vectorPop(first_ptr);
+        l->size -= 1;
 
-    if (l->size <= l->capacity / 2) l->capacity /= 2;
-    if (l->capacity == 0) l->capacity = 1;
+        if (l->size <= l->capacity / 2) l->capacity /= 2;
+        if (l->capacity == 0) l->capacity = 1;
 
-    return ptr;
+        return ptr;
+    } else {
+        // cannot pop when there is no value in the list
+        return NULL;
+    }
 }