Move Rust build into its own flake at rust/flake.nix using crane, and
reference it as an input from the top-level flake. The Haskell build
gets the Rust artifacts (header + static lib) via a haskell.nix
override that symlinks them into place before configure.
Use Cabal's Cgarnet_rs naming convention for extra-bundled-libraries
to satisfy the shared library install phase naming requirements.
Also extract inputs.nix, default.nix, and shell.nix for flake-compat
(nix-build / nix-shell) support.
See https://sraka.xyz/posts/hs-bindgen-introduction.html.
For now, this is a shell-based workflow, rather than using Nix to build everything, i.e. `nix develop` works but `nix build` doesn't. And `cargo build` has to be called manually to create the C library, rather than `cabal` being clever enough to invoke it itself.
We ran `cargo cabal init` from the `rust` directory (`nix shell github:yvan-sraka/cargo-cabal`), which generated `hsbindgen.toml` (which we use), and `Setup.lhs` (which just added `extra-lib-dirs`, and with the wrong paths, so we dspecify those statically instead in `garnet.cabal`). We also follow its advice to use `staticlib`.
Also, after we ran the first `cargo build` (requiring a `mkdir rust/src` before it would run), we took the generated the Haskell file, and moved the main contents in to `Main.hs` manually.
