Use Nixpkgs Wasm GHC

[georgefst]

See various TODOs.

This is a sketch of what it would look if we had GHC Wasm support in Nixpkgs, and could thus easily have an actual Nix derivation for the Wasm Coln compiler. This does work, but relies on a somewhat vibe-coded Nixpkgs branch whose GHC currently has to be built from source. I will make some attempts to clean up and upstream that work, but it's moderately complex and there's no guarantee of it landing any time soon. I'm just leaving this here as a proof of concept.

[georgefst]

Oh whoops, I'd forgotten to force-push the version that commented out the YAML lines to avoid a huge GHC build. Looks like someone manually killed the build after 20 minutes...

[georgefst]

We've tried this same Nixpkgs branch out on another Haskell Wasm project at Obsidian and found that for very large Haskell libraries we hit a fatal linker error which means that we will need some of ghc-wasm-meta's LLVM patches, at least this one. This significantly reduces the chances of getting to a mergeable Nixpkgs PR any time soon.

[georgefst]

This significantly reduces the chances of getting to a mergeable Nixpkgs PR any time soon.

Meanwhile, if anyone is interested in following progress on this, take a look at this branch. It might be that the compiler we had to stop CI from building from source in this PR will soon turn up in Reflex's Nix cache.