Splice ./universe directory into ./

Manually merging:
- README.md: I added the description from universe/README.md into the heading of
  dotfiles/README.md.
- .envrc: dotfiles/.envrc was a superset of universe/.envrc
- .gitignore: Adding some of the ignored patterns from universe/.gitignore to
  dotfiles/.gitignore

Everything else here should be a simple rename.

universe/advent-of-code/writePythonBin.nix

@@ -1,18 +0,0 @@
-{ pkgs ? import <nixpkgs> {}, ... }:
-
-{ name, deps, src }:
-
-let
-  inherit (pkgs) pythonPackages writeTextFile;
-  inherit (builtins) toFile;
-
-in writeTextFile {
-  inherit name;
-  executable = true;
-  destination = "/bin/${name}";
-
-  text = ''
-    #!/bin/sh
-    ${pkgs.python3}/bin/python3 ${src}
-  '';
-}