Use scala.collection.immutable.Queue for FIFOQueue instead of List

scala.collection.immutable.List is a singly linked list, so it is not efficient(O(n)) to append elements.
Using scala.collection.immutable.Queue, enqueue will be faster.
Merge pull request #3 from okumin/fix-data-structure

Use scala.collection.immutable.Queue for FIFOQueue instead of List