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/split-sequence.nix

@@ -0,0 +1,18 @@
+# split-sequence is a library for, well, splitting sequences apparently.
+{ pkgs, ... }:
+
+let src = builtins.fetchGit {
+  url = "https://github.com/sharplispers/split-sequence.git";
+  rev = "41c0fc79a5a2871d16e5727969a8f699ef44d791";
+};
+in pkgs.nix.buildLisp.library {
+  name = "split-sequence";
+  srcs = map (f: src + ("/" + f)) [
+    "package.lisp"
+    "vector.lisp"
+    "list.lisp"
+    "extended-sequence.lisp"
+    "api.lisp"
+    "documentation.lisp"
+  ];
+}