tree: fix broken tests, and improve doc-comments (Fixes #40)

* specs/tree_spec.yaml (inodes, nodes): Be sure to clone iterator
returned paths before storing. Some corrections to the expected
results.
Remove pending "see issue #40" instances.
(merge): Fix tests. Remove pending "see issue #40" instances.
* ext/std/tree.lua (nodes): Much improved doc-comment.
(merge): Assert the arguments are actual tree nodes, because
raw tables do not work!

Signed-off-by: Gary V. Vaughan <gary@gnu.org>