Thanks Stefan for the patch. It's very much appreciated. I will try to review it soon.

Of the features you proposed, the twos I would like to take a look again is implicit memoization and the BAIL_OUT opcode. For the implicit memoization feature, we will need to have some performance results in hand to justify the major changes it needs. If you can you work out a quick patch, I can run it through the benchmarks suite for pickle and measure the impact. Hopefully, we will see a good improvement though we can't be sure until we measure.

And as for the BAIL_OUT opcode, it would be interesting to revisit its use now that we support binary framing. It could be helpful to add it to prevent the Unpickler from hanging if the other end forgot to close the stream. I am still not totally convinced. However if you make a good case for it, I would support to see it included.