Raise exception when rendering a generator twice

[pelme]

In #101 @JakobGM pointed out that creating an element with a generator is a foot gun that can lead to unexpected results.

Instead of silently rendering empty elements, we should raise an error when trying to render a element with a generator child node the second time.

A test could be something like:

def test_raise_error_consume_generator_twice() -> None:
    def gen() -> Iterator[str]:
        yield "hi"

    element = div[gen()]
    assert list(element.iter_chunks()) == ["<div>", "hi", "</div>"]

    with pytest.raises(RuntimeError):
        list(element.iter_chunks())

the implementation could probably be setting a boolen attribute _generator_consumed = True on nodes that has a single generator child node and then checking for that existence (in _iter_chunks()).

This is slightly backwards incompatible but I would consider code that relies on this behavior to already be broken so I think we can just change this. Feel free to submit a PR for this!