refactor: move memoize back to std.functional.

* specs/std_spec.yaml (memoize): Move from here...
* specs/functional_spec.yaml (memoize): ...to here.
* lib/std.lua.in (memoize): Move from here...
* lib/std/functional.lua (memoize): ...to here.
* NEWS: Update.

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