Colocate Traversal classes with classes they traverse

This puts all knowledge of the traversal in the same file, and removes
the need for a CASES registry.

lib/liquid.rb

@@ -45,6 +45,7 @@ module Liquid
 end
 
 require "liquid/version"
+require 'liquid/traversal'
 require 'liquid/lexer'
 require 'liquid/parser'
 require 'liquid/i18n'