Fix nodes-equalp to recursively compare the branches of a node.

Previously, nodes-equalp used equal to compare branches, which was
causing three of the lift tests to fail.
Use optional STREAM argument in print-words.

Although print-words accepts an optional STREAM argument, it was always
printing on *standard-output*.
Merge pull request #2 from appleby/print-words-to-stream

Fixes for print-words and nodes-equalp