Implement suggested performance improvements for `text.find` / `grep`

[pchiusano]

See comments from @ChrisPenner on this PR https://github.com/unisonweb/unison/pull/5362 which I saw after it had already been merged. They all sound good to me but I'm swamped at the moment. Someone else want to take this?

[mitchellwrosen]

Sure, I can pick it up

[pchiusano]

Thank you!!