functional: make nop an official functional method.

* specs/functional_spec.yaml (nop): Specify behaviour of new nop
module method.
* specs/spec_helper.lua.in (nop): Remove one-off nop declaration.
* specs/string_spec.yaml (finds): Specify in-situ nop.
* specs/base_spec.yaml (std.base): Likewise.
* lib/std/base.lua (nop): Declare an official nop function.
* lib/std/functional.lua (nop): Re-export std.base.nop.
* NEWS: Update.

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