Add foldable transformers

[akeep]

It is often the case that we want to fold a transformer over a list of nonterminals and produce the resulting list of nonterminals and another folded result.

It is not entirely clear how this should work, since in some cases we'd want to just create a list of nonterminals (like the current map does), however, sometimes we would like to fold over that list as well.

This is a boilerplate elimination enhancement, so it is worth considering what the right point in the space is.