PostOrder: foldl and foldr'

In the course of #1244 I realized that `foldl` and `foldr'` can be productive for `PostOrder`. I'll take a look at how well the default implementations work, and implement it manually if needed.

Read issue #1244 for context, then inspect the PostOrder entry points for foldl and foldr' and how their default implementations behave. Done means confirming whether both folds are productive for PostOrder and implementing them manually if the defaults are insufficient.

Written by the indexing model from the issue text.