Nix Support: package exchain for default.nix & flake.nix

[Kodylow]

Summary

I'd like exchain to be packaged for Nix distribution.

Problem Definition

Exchain does not support nix distribution so cannot be effectively run inside of Replit's in-browser IDE.

Proposal

Nix is a package management system for cross platform deterministic builds, and is integrated with Replit so any project packaged for Nix can be pulled into Replit's in browser IDE for development with exchain without having to do any of the golang or rust setups. I demoed this to @KamiD yesterday.

I'm working on doing the nix packaging myself and will do a PR once I get it working locally. This requires no changes to exchain, it's just adding a default.nix file and a flake.nix file with the nix recipes for installing go, rust, rocksdb, and building the exchain binaries. It's similar to adding support to a new UNIX system.

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