maint: use "int" argcheck type as appropriate.

* lib/std/array.lua, lib/std/functional.lua: Use "int" parameter
type with argcheck as appropriate.
* lib/std/debug.lua (argscheck): Adjust usage example.
* specs/array_spec.yaml, specs/functional_spec.yaml: Adjust.

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