It could be interesting to benchmark the starting time with and without this change, see if loading IPython affects start time.

This is just a simple test for now. Only code execution works with the display hook.

This could allow getting rid of a lot of code in xeus-python: error traceback, inspection, completion, etc. It would also bring magics support for free, even though some work would still be required to support matplotlib.

What remains to have a mergeable PR:

• Keep user_ns as a C++ interpreter attribute
• Find a suitable approach for executing Python code without using shell.run_cell (for the debugger and xeus-robot) while getting back the traceback correctly formatted and updating the linecache. We can get rid of xtraceback.cpp if we find a proper solution with IPython.
• Custom display hook. (Check that SlicerJupyter's display hook can still work with the new approach)
• Hook clear_output

Wow, this is a really big step for usability. Awesome work!

Epic!