refactor: move list.depair and list.enpair into std.table.

* specs/table_spec.yaml (depair, enpair): Specify full behaviours.
* specs/list_spec.yaml (depair, enpair): Specify deprecation
warnings.
* lib/std/list.lua (depair, enpair): Move from here...
* lib/std/table.lua (depair, enpair): ...to here.
* NEWS: Update.

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