Use buildRustPackage instead of buildRustCrate (via crate2nix).
buildRustPackage builds the whole executable and its libraries in a
single `cargo build` process.
With the create2nix approach each library is built in a separate derivation,
directly using rustc instead of the cargo wrapper.
Benefits of buildRustPackage:
- Much simpler to maintain
- Package derivation evaluates much faster
Benefits of crate2nix:
- Build can be distributed over multiple build hosts
- Better sharing of common dependencies between different builds
- More fine-grained rebuilding on build failures
In nixpkgs buildRustPackage is used for almost all Rust pkgs, it's
also a better fit for our use case.
