Allow different crate and library names, R pacakges with dots in their names

[Ilia-Kosenkov]

• Fixes #176, allowing R package names like {test.package}
• Fixes #152, allowing to set crate name and library name (= the name of the binary produced by Rust) separately from the name of the R package, resolving potential crate/package name collisions.

'Names' we distinguish:

• ✔ R package name, sanitized if necessary. This is the name of the final binary artifact and the name of the topmost module defined in src/rust/lib.rs;
• ✔ Rust crate name, which defaults to the sanitized R package name. May be helpful in resolving conflicts with cargo dependencies;
• ✔ Rust library name, which defaults to the sanitized R package name. Same usage as described above;
• ❌ We can potentially distinguish between R package name and topmost exported Rust module, but this may need some support from extendr If the module name differs from that of the R package, we will have to pass this name to {rextendr} every time we call rextendr::document(), because this name determines the name of the function that produces wrappers. I suggest to not support this feature.

TODO:

• [x] Check docs
• [x] Run {styler}
• [x] Write tests for package/crate name combinations

[Ilia-Kosenkov]

I think I covered all cases.

[Ilia-Kosenkov]

I think everything has been resolved. I am going to merge this if there are no objections.

[yutannihilation]

Looks good, in case you need another explicit approval!