refactor: move `table.metamethod` to `std.getmetamethod`.

Be more in keeping with the style of core Lua.
* lib/std/table.lua (metamethod): Deprecate.
* lib/std.lua.in (getmetamethod): Export from here instead.
* specs/table_spec.yaml, specs/std_spec.yaml: Adjust accordingly.
* NEWS: Update.

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