Restructure recursors and add some lens interfaces #1489
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This PR restructures the recursors in two categories: Applicative and Monadic.
In Applicative we find folds and maps to the leaves. In Monadic we find maps (top-down and bottom-up).
The advantage of having Applicative folds is that they can play well with some lens interface (e.g.
Traversal,SimpleFold).As an illustrative example, this PR implements a
SimpleFoldover free variables inside a node, and aTraversalover all the identifiers. One can then easily use the existing tools from the lens ecosystem to define a function to determine whether a node is closed, as shown in this PR.Having a lens interface is not always possible, however, I think that for the cases where it is, we should try to have it.