flake.nix: use crate2nix

Also switch to nixos-20.09, and add a shell.nix and default.nix that wrap the flake.

Created by  jwaksbaum, cole-h  on November 16, 2020
G734WNM64AR5BLAZMN5MDPKSFTYXTUQR6MAGB32NRBC5FXFRWSJAC
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