tree: allow objects as keys.

* lib/std/tree.lua (Tree.__index): Only fold key list when key
parameter is a raw list, and not when it is a std.object derived
table.
(Tree.__newindex): Only descend the key list creating sub-Trees
when key parameter is a raw list, and not when it is a std.object
derived table.
* NEWS: Update.

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