Renased Ngram

I would put this big `for` as an independent function like `_term_to_edge_ngram` that `yield`s terms (see [`_term_to_xapian_value`](https://github.com/notanumber/xapian-haystack/blob/master/xapian_backend.py#L1468)).

This is because, like [`_from_xapian_value`](https://github.com/notanumber/xapian-haystack/blob/master/tests/xapian_tests/tests/test_backend.py#L591), we need to test that the procedure is giving the expected terms.