Looping in Julia programs and traces

As a followup to #52, we should investigate whether loops in Julia programs can be converted into traces (feedback) in the wiring diagrams. In a cartesian monoidal category, traces are the same as fixed point operators and so are closely related to recursion and looping.

This question was raised on [Twitter](https://twitter.com/laMudri/status/1198003421758021634). Also mentioned was Conor McBride's [Syrup](https://github.com/pigworker/Syrup/blob/f5ba570e874e845ecdad52e98d31dfbb80da6394/doc/Syrup.pdf), a pedagogical programming language for Boolean circuits with a strong flavor of monoidal categories. However, I can't find anything in the Syrup documentation about looping.