Bump `smallvec` dependency to avoid issues with mem::uninitialized in newer rust

Hello! In rust-lang/rust#99389 we're making the checks for mem::uninitialized stricter, and old versions of smallvec run into them.

You can just upgrade smallvec to the latest stable to avoid this, it seems like there were no breaking changes in the bits of the API that you use, tests seem to pass fine on version 1.9.0.