Change gc for nim's fast implementation to recommended arc gc#87

While markAndSweep is mostly better than the default gc in nim, the new "arc" gc is even more performant and results in a lot less memory usage overall. In order to be consistent with new upstream development, the "fast" nim implementation should use arc. More details available at https://nim-lang.org/docs/gc.html See https://youtu.be/yA32Wxl59wo?t=830 for an example benchmark. Using arc instead of markAndSweep saves over 10 seconds and uses less memory!