functional: filter supports multi-parameter predicates.

* specs/functional_spec.yaml (filter): Specify behaviours when
called with an iterator that returns multiple values, and
predicate that accepts multiple parameters.
* lib/std/functional.lua (filter): Collect all results from
iterator function, and propagate them all to the predicate call.
* NEWS: Update.

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