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.

advent-of-code/writePythonBin.nix

@@ -0,0 +1,18 @@
+{ 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}
+  '';
+}