D4118: index: make node tree a Python object

[Yuya Nishihara]

> +	PyObject *index;
> +	unsigned capacity;
> +	if (!PyArg_ParseTuple(args, "OI", &index, &capacity))
> +		return -1;
> +	return nt_init(self, (indexObject*)index, capacity);

One more thing, we'll probably need to enforce the index type, by `"O!"`.

And a lookup of `0xxx...` hash might not work well due to the nullid entry.
I recalled it while I was making a breakfast. I might be wrong.