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.

third_party/lisp/cl-arrows.nix

@@ -0,0 +1,15 @@
+{ depot ? import <depot> {}, ... }:
+
+let
+  src = builtins.fetchGit {
+    url = "https://github.com/nightfly19/cl-arrows.git";
+    rev = "cbb46b69a7de40f1161c9caaf6cef93b3af9994f";
+  };
+in depot.nix.buildLisp.library {
+  name = "cl-arrows";
+  deps = [];
+  srcs = [
+    "${src}/packages.lisp"
+    "${src}/arrows.lisp"
+  ];
+}