refactor: move list.flatten to table.flatten.

* specs/list_spec.yaml (flatten): Specify deprecation warnings.
* specs/functional_spec.yaml (flatten): Move from here...
* specs/table_spec.yaml (flatten): ...to here.
* lib/std/list.lua (flatten): Move from here...
* lib/std/table.lua (fatten): ...to here.
* lib/std/functional.lua (collect): Move core from here...
* lib/std/base/functional.lua (collect): ...to here.

* specs/list_spec.yaml (flatten): Deprecated properly.
* lib/std/list.lua (flatten): Wrap object method in deprecation
warning.

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