Add a method to find paths to Ractor-unshareable objects which can be
traced from an object.

Example:

    class Container
      attr_reader :value
      def initialize(value)
        @value = value
      end
    end

    mutable_string = "hello"
    container = Container.new(mutable_string)

    pp ObjectSpace.find_paths_to_unshareable_objects(container).to_a
      #=> [
        [#<Container:0x00007fc35843e388 @value="hello">],
        [#<Container:0x00007fc35843e388 @value="hello">, "hello"]
      ]

Co-authored-by: Yusuke Endoh <mame@ruby-lang.org>