Potentially unneeded memory alloc in `StopwordCount`

I don't think we need to create a list in this comprehension:
https://github.com/alteryx/nlp_primitives/blob/11837a50de79fd05e067f58075100f39f639e563/nlp_primitives/stopword_count.py#L42

Instead, I think we can just iterate over `words` and keep a `count` variable. For very large strings this would save the time and memory needed to alloc the list.