use_version()-like function to sync package version between R and rust crate

[JosiahParry]

The default package version for an R package using usethis is 0.0.0.9000 and the default package version for the rust source library is 0.1.0. Maintaining versioning between the source and the R package can be easy to forget. It would be great if there was an rextendr function that could modify the Rust crate version along with the R package version. It could use tomleditR and usethis.

[eitsupi]

Related to #228

[JosiahParry]

Thanks! Re:

"I think rextendr should not modify other than then extedr-wrappers.R. Cargo.toml is supposed to be edited by the package author, so trying to modify it might cause unexpected results."

When I use use_version() from {usethis} I am knowingly invoking the function and doing it manually. As long as this isn't part of some automated process such as document(), I don't think it would be invasive.

[eitsupi]

Probably better to leave the updating of Cargo.toml to something like cargo-release than to implement this in R.
https://github.com/crate-ci/cargo-release

[JosiahParry]

Perhaps, if we do not want to conflict with usethis, the function could be something like sync_crate_version() which could take the version in the DESCRIPTION file and use it in the Cargo.toml.