Fix off-by-one in Vector (the probable cause for many user-reported crashes?)

Vector.c

@@ -133,8 +133,8 @@ void Vector_insert(Vector* this, int idx, void* data_) {
    
    Vector_checkArraySize(this);
    assert(this->array[this->items] == NULL);
-   for (int i = this->items; i >= idx; i--) {
-      this->array[i+1] = this->array[i];
+   for (int i = this->items; i > idx; i--) {
+      this->array[i] = this->array[i-1];
    }
    this->array[idx] = data;
    this->items++;