set: derive from std.container, keys no longer clash with method names.

* specs/set_spec.yaml: Replace object methods with prototype
functions through out.
Check that set elements with the same name as a prototype
function behave properly.
* lib/std/set.lua: Rewrite as a derivative of std.container.
Change object methods to Container style prototype functions.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
refactor: automatically fill method functions form _functions table.

* lib/std/container.lua (metatable.__call): Copy entries from
newly passed _functions table to object.
* lib/std/set.lua (Set): Delete manual copy of method functions.

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