def cache_bust(target)
  class << @site
    attr_accessor :cache_bust_map
  end
  @site.cache_bust_map ||= {}
  @site.cache_bust_map[target] ||= begin
    require 'xxhash'
    if target.is_a? String
      target = @items[target]
    end
    relativize_path(target) + "?#{XXhash.xxh32(target.compiled_content, target.checksum.to_i)}"
  end
end