feat(third_party/lisp): Check in drakma HTTP client & deps

This HTTP client has much fewer dependencies than some of the other
ones I've seen, and it'll be good enough for my demo purposes.

third_party/lisp/usocket.nix

@@ -0,0 +1,35 @@
+# Usocket is a portable socket library
+{ pkgs, ... }:
+
+let src = pkgs.third_party.fetchFromGitHub {
+  owner = "usocket";
+  repo = "usocket";
+  rev = "fdf4fd1e0051ce83340ccfbbc8a43a462bb19cf2";
+  sha256 = "0x746wr2324l6bn7skqzgkzcbj5kd0zp2ck0c8rldrw0rzabg826";
+};
+in pkgs.nix.buildLisp.library {
+  name = "usocket";
+  deps = with pkgs.third_party.lisp; [
+    asdf
+    sb-bsd-sockets
+    split-sequence
+  ];
+
+  srcs = [
+    # usocket also reads its version from ASDF, but there's further
+    # shenanigans happening there that I don't intend to support right
+    # now. Behold:
+    (builtins.toFile "usocket.asd" ''
+      (in-package :asdf)
+      (defsystem usocket
+        :version "0.8.3")
+    '')
+  ] ++
+  # Now for the regularly scheduled programming:
+  (map (f: src + ("/" + f)) [
+    "package.lisp"
+    "usocket.lisp"
+    "condition.lisp"
+    "backend/sbcl.lisp"
+  ]);
+}