fix: fixes issue #597 - improves performance (500x faster than previous)

[a-h]

No description provided.

[joerdav]

All the code looks solid to me, I also tested it on my shopping-list repo, which doesn't really benefit form the performance increase much, but it does still work.

One thing with these open/close counters is I get skittish about some scenarios like f("hello world{"), where strings contain open or close brackets. Looking at the code I think we're fine, but could do with a few test cases to be positive.

Would also probably be an ideal time to run the fuzz tests again!