wip: rewrite s6-rc-up-tree in an actual procgramming language

and write some tests for it, too

pkgs/s6-rc-up-tree/test.fnl

@@ -0,0 +1,14 @@
+(local up-tree (require "s6-rc-up-tree"))
+
+(os.remove (os.getenv "TEST_LOG"))
+
+(let [[dir & services] arg]
+  (set arg services)
+  (up-tree.run dir))
+
+;; the service starts
+;; the service starts even if it is controlled
+;; uncontrolled descendants start
+;; controlled descendants don't start
+;; descendants which depend on a _different_ controlled service, which is down, don't start
+;; descendants which depend on a _different_ controlled service, which is up, do start