Suggestion: add label for callable nodes

I've been familiarizing myself with yap. As I've been implementing new callable structs, it has been helpful to add a label to the struct that is handled by yap::print if present. Is this of general interest or is there a better way to accomplish this task?

Start by reviewing the callable structs handled by yap::print and the surrounding expression-printing entry points. Clarify whether labels should be supported generally, how a label is declared and rendered, and what tests would demonstrate the intended behavior before implementing anything.

Written by the indexing model from the issue text.