Add fallback when uuid is not available

If the `uuid` module does not exist, fall back to a primitive ID
generation mechanism based on just math.random().  The fallback
mechanism provides the same amount of entropy so we can be sure this
won't degrade quality of the identifiers.

bc.lua

@@ -1,4 +1,3 @@
-local uuid = require("uuid")
 local bit32 = require("bit32")
 
 local Version = {0, 1}
@@ -40,6 +39,23 @@ do
     clock = os.clock
   end
 end
+
+local next_uuid -- Generate a UUID
+do
+  local status, uuid = pcall(require, "uuid")
+  if status and uuid.next ~= nil then
+    next_uuid = uuid.next
+  else
+    -- Fallback to something reasonably close
+    next_uuid = function()
+      local uuid = ""
+      for _ = 1,16 do
+        uuid = uuid..string.format("%02x", math.random(0, 255))
+      end
+      return uuid
+    end
+  end
+end
 -- }}}
 
 -- Network ---------------------------------------------------------------- {{{
@@ -420,7 +436,7 @@ function BaseControl:listen(noun, query, callback)
   if self.listeners[noun] == nil then
     self.listeners[noun] = {}
   end
-  local id = uuid.next()
+  local id = next_uuid()
   self.listeners[noun][id] = {
     query=query,
     callback=callback,