Implement "parameter punning" for the code notation `%cd`

[lukstafi]

This is really for show-off purposes. But it's just too tempting to not use it in the upcoming OCaml Workshop paper, and it adds extra consistency with the %op notation.

[lukstafi]

Let's have it also populate the label with the labels collected from the right-hand-side of the punning line. Then we could actually use it in the Train.sgd_one code!