Support for other Ruby implementations

[eregon]

Deep Clone seems to heavily depend on internals that MRI itself does not expose.

I think the best way to support other Ruby implementations such as TruffleRuby and JRuby would be to use a pure-Ruby fallback using Marshal.load Marshal.dump(obj) on any implementation where RUBY_ENGINE != "ruby".
Do you think this could be done in this gem?
Then it would work transparently on other Ruby implementations.

From truffleruby/truffleruby#2073