functional: new callable module function.

* specs/functional_spec.yaml (callable): Specify correct behaviour
for new callable function.
* lib/std/functional.lua (iscallable): Move from here...
* lib/std/base/functional.lua (callable): ...to here.  Adjust all
callers.
* lib/std/functional.lua (M.callable): Reexport as a public api.
* NEWS: Update.

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