Should we use Apache Calcite or Substrait?

PuffinDB will include a distributed query planner. This complex component should probably be developed on top of an existing framework implementing a [relational algebra](https://en.wikipedia.org/wiki/Relational_algebra). The two main options are [Apache Calcite](https://calcite.apache.org/) and [Substrait](https://substrait.io/). Which one should we use? Either should work, but we must pick one. This decision will have major and lasting impacts on the codebase.