WIP draft of complex-numbers concept

[colinleach]

A rather self-indulgent concept: not quite up there with arrays and loops as a priority concept, but I wrote the Python version with @BethanyG a few months ago, and it translated pretty smoothly to Julia.

As with rational-numbers, we discussed some vague ideas for exercises but didn't reach a decision.

It would be great if we could think of something, so we could use it for both Julia and Python. Sadly, my usual plan of stealing exercises from Elixir or Go doesn't work for the more mathematical stuff.