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.
This reverts commit 1f1c0d959da699ce04f7951ecbcdb7976c8c0750.
This doesn't work well with multi-component builds. For example, it requires `"haskell.sessionLoading": "singleComponent"` in VSCode, which makes HLS work less reliably.
The script addition is a bit hacky, but there's no obvious straightforward arch/version-independent way to get most of the build path. And eventually, once issues with HLS etc. are sorted out we will revert to using Cabal hooks anyway.
Even though `Int` and `isize` should be the same in practice, we can't cleanly convert, as the type information isn't quite there. And anyway, strictly speaking per the report, `Int` is only guaranteed to hold 30 bits.
Note that this is essentially unchanged even if we specify `usize_is_size_t = true` for `cbindgen`.
We drop the tools specifically designed for Haskell and Rust together, in favour of general tools for using each with C.
Namely, we use Mozilla's `cbindgen` for generating header files from the Rust source, and Well-Typed's new `hs-bindgen` tool for generating Haskell from those header files.
The Rust code here is essentially the result of expanding the old macro, then inlining and renaming internals.
The most important thing here is that we're now relying solely on robust well-maintained tools.
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.
