feat: flake.nix compliation and devShell #2

Adding a nix flake that perhaps is a little over-engineered for this, but does correctly build and run pdsfs. The flake exports a package and devShell, but not an overlay. Optionally can use latest rust using the rust-overlay to build if you want with the nightly-devShell. Only other dependency is flake-utils, but can remove. Haven't tested on any platform other than linux.