Build and call Rust code from Haskell

Description

This is just an experiment to demonstrate that:

• A Haskell package can call Rust code
• The Cabal package can be configured to automatically build the Rust dependency

Commands

• nix build . builds the Haskell program.
• nix build .#rs_hello_world builds the Rust library.
• nix develop enters the development Shell for the Haskell program.