exp_string should take String.

I don't know how ppl feel about API changes, I'd guess there are so few users it doesn't matter as long as the minor version id bumped appropriately.

APIs should take owned values when they need an owned clone of something, instead of calling `to_owned()` or in this case `to_string()`.

See: https://www.philipdaniels.com/blog/2019/rust-api-design/

