Handle `user_expressions`

[arnsholt]

The user_expressions entry in the execute request is IPython's mechanism to allow the frontend to request additional stuff to be computed, which powers things like dynamic prompts. We don't support that yet.

Implementing this feature should be relatively straightforward; it's mostly a question of extracting the relevant things from the message and putting it in the right place in the response. Extracting the "evaluate code" bits from !execute into a separate method is likely a good refactoring to share this between both code paths.