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