table: ensure there is always a maxn function.

* specs/table_spec.yaml (maxn): Specify behaviour of maxn.
(len): Add missing specifications.
* specs/std_spec.yaml (barrel): Add maxn to list of monkey_patch
apis from std.table.
* lib/std/base.lua (maxn): Use core table.maxn if available, or
else define our own.
* lib/std/container.lua, lib/std/debug.lua: Use it!
* lib/std/table.lua (maxn): Export it.
* NEWS: Update.

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