This ticket is intended just to capture blockers for starting a full-stack cleanup of the pybind11 branch prior to merge. That phase will involve two kinds of work we don't want to start too early:

• Small, minimally disruptive changes to existing Python or C++ APIs that greatly improve the pybind11 wrappers (generally in terms of code simplicity or performance). We don't want to make these changes without having very good test coverage in place.
• Global changes to how we define wrappers that we don't want to make twice.

Therefore, before that cleanup phase begins, we want to:

• have our coding conventions for pybind11 fully settled so we don't need to revisit files again later;
• have any utility code required by the conventions in place;
• have as many contributed-upstream pybind11 features in place as possible;
• have enough of the stack wrapped to handle high-level integration testing (particularly ci_hsc).