Packaging Rust crates in GNU Guix

• Not much people who're familiar with Rust, apparently (at least at the time of the talk). Nonetheless, Rust-based tools are included.

• Created an importer using the Crates.io API. With the importer, it can recursively build the module all the way down (most of the time).

• It pulls all of the dependencies. The side effect is that every package has to be defined. The importer just makes it easier.

• The community have a preference to shared libraries and sources. Vendoring is not much of a popular option. Since Rust downloads everything and the community wants to reuse the components, it seems the solution is to package everything.