Add cargo_crate repository rule

[davidzchen]

Add a cargo_crate repository rule for fetching Rust crates from Cargo.io.

[UebelAndre]

Understanding there's a sort of bootstrapping issue, I think it'd still be worth getting the code in. Maybe BuildKite has a way to build artifacts and commit them back to the repo?

[illicitonion]

I think it's time to open a PR for this then 😄 So we can appropriately comment on the changes.

I just created https://github.com/bazelbuild/rules_rust/pull/598 adding the crate_universe rule to //cargo:repositories.bzl :)

[UebelAndre]

@illicitonion @hlopko I think this issue could be closed now that https://github.com/bazelbuild/rules_rust/pull/598 is in

[hlopko]

Makes sense to me. Please complain if anybody disagrees :)