Packaging Rust for Fedora

Note that my idea (that I've posted about before) is that installing a Rust package puts its crate into a machine-local registry. This would allow for automatic resolution to distro-provided versions based on `BuildRequires:` and the like.

Note that just rehosting `crates.io` might not be preferrable since they can be post-codegen steps (of various kinds) and instead `cargo publish` into this registry that is then pulled out during some `rpm-cargo-*` macro magic would probably be a better mechanism.