Folds over the substructure of a type, visiting its component types and all regions that occur free within it.

The TypeFoldable trait is implemented for every type that can be folded. Basically, every type that has a corresponding method in TypeFolder.

The TypeFolder trait defines the actual folding. There is a method defined for every foldable type. Each of these has a default implementation that does an "identity" fold. Within each identity fold, it should invoke foo.fold_with(self) to fold each sub-item.