table: add elems and ielems module functions.

* specs/table_spec.yaml (elems, ielems): Specify behaviour of
new iterators.
* lib/std/table.lua (elems): Iterate over all values of a table,
for orthogonality with std.list and std.set.
(ielems): Expose base.ielems in a type checking wrapper.
* NEWS: Update.

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