refactor: move `metamethod` from `functional` to `table` module.

* lib/std/functional.lua (metamethod): Move from here...
* lib/std/base.lua (metamethod): ...to here; and...
* lib/std/table.lua (metamethod): ...re-export from here.
Break dependency on `std.functional`.
Adjust all callers.
* NEWS: Update.

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