Update handlers.lua

`Handlers.once` does a `prepend`: these should be higher priority than fallback handles. 

Use case: worker pool.
- Scenario 1: The master process resets a workers "busy" status upon receiving a "Ready" message.
- Scenario 2: When the all workers are busy, the master process should suspend by doing `Receive` on (any) workers "Ready" message. Resuming the thread should take priority over resetting the status as in Scenario 1

process/handlers.lua

@@ -80,7 +80,7 @@ function handlers.once(...)
     pattern = select(1, ...)
     handle = select(2, ...)
   end
-  handlers.add(name, pattern, handle, 1)
+  handlers.prepend(name, pattern, handle, 1)
 end
 
 function handlers.add(...)
@@ -305,4 +305,4 @@ function handlers.evaluate(msg, env)
   end
 end
 
-return handlers
+return handlers