specs: improve expectation description.

* specs/set_spec.yaml: Write "shows the type name" instead of
"contains the type".

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
doc: update object constructor _function documentation.

* lib/std/container.lua: Update module doc-comment to reflect
simplification of passing module functions in _functions table.
(std.container): Update _function description.
* lib/std/object.lua (std.object): Likewise.
* lib/std/set.lua (std.set): Likewise.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
tree: derive from std.container.

* specs/tree_spec.yaml: Replace object methods with module
functions throughout.
Add specs for inheritted functionality.
* lib/std/tree.lua: Rewrite as a derivative of std.container.
Change object methods to Container style module functions.
* NEWS: Update.

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