table: add merge_select, and support map and nometa args to merge.

Close #56.
* specs/table_spec.yaml (extend_base): Add merge_select.
(merge): Specify behaviour of new `map` and `nometa` args.
(merge_select): Specify behaviour of new `merge_select` api.
(clone, clone_select): Refactor for clarity and orthogonality.
* lib/std/base.lua (merge): Rewrite to support clone-like `map`
and `nometa` parameters, according to improved specifications.
(clone): Rewrite as a call to `merge`.
* lib/std/table.lua (merge_select): New `clone_select` like api
satisfying specs.
(clone_select): Rewrite as a call to `merge_select`.
* NEWS: Update.

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