Switch from c2hs to hs-bindgen

[georgefst]

To be squash-merged...

This is now looking pretty good, apart from two remaining hacks:

• Hardcoded Nix store paths in the TH generation code.
• We're using a messy vibe-coded Generator.hs to produce Codes.hs. We should clean this up if keeping it. Otherwise maybe hs-bindgen's CEnum stuff could be adapted to be used with macro-based enums. There'd have to be some way to specify the grouping of the constants in to separate types, e.g. by newline-separated block or name prefix.