refactor: move case back to std.functional.

* specs/std_spec.yaml (case): Move from here...
* specs/functional_spec.yaml (case): ...to here.
* lib/std/base.lua (case): Move implementation from here..
* lib/std.lua.in (case): ...and argcheck wrapper from here...
* lib/std/functional.lua (case): ...to here.
* lib/std/package.lua (path_sub): Decouple from std.functional by
comparing manually rather than using functional.case.

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