rust linter
https://github.com/adsharma/py2many/issues/266 slipped in because clippy isnt being run.

When I was adding linters, I didnt find an easy way to run `clippy` on a single file. c.f. https://github.com/rust-lang/rust-clippy/issues/5613

Maybe there is some other linter we might use.

Otherwise we need to create a Cargo.toml, etc to make cargo happy to run clippy on the single file.