Make the debug gem work on TruffleRuby

[eregon]

This is more of a FYI issue. The TruffleRuby devs are looking at running this gem test suite and try to make it work on TruffleRuby. Ideally it would require no or very few changes in this gem, as the gem only uses official Ruby APIs + C extensions from what I saw.

The first item is getting the pty stdlib to work on TruffleRuby since that's used by the test suite: https://github.com/oracle/truffleruby/issues/1589 Then TruffleRuby will likely need to support more of TracePoint e.g. https://github.com/oracle/truffleruby/issues/1672. After that I would think it's mostly small fixes.

Feel free to close issue if you want to keep open issues only for "things to address in this gem".

[eregon]

One extra requirement is https://github.com/ruby/debug/issues/887