Fixed node content printing in make-folding-node-printer (again) in src/print-tree.lisp

* src/print-tree.lisp (make-folding-node-printer): when determining how
  to print the node content, take into account whether there are
  children and whether printing children has been requested
Minor refactoring in make-folding-node-printer in src/print-tree.lisp

* src/print-tree.lisp (make-folding-node-printer): moved check for
  whether to print children
More minor refactoring in src/print-tree.lisp

* src/print-tree.lisp (print-nodes): renamed local variable element ->
  node
  (make-node-printer): likewise
  (make-folding-node-printer): likewise