functional: rename fold to reduce.

* specs/functional_spec.yaml (fold): Remove argument checking
specifications.
Add deprecation warning specification.
(reduce): Specify identical behaviour to old fold api.
* lib/std/functional.lua (fold): Rename from this...
(reduce): ...to this.
(fold): Show a deprecation warning on first use.
* specs/container_spec.yaml (construction): Adjust.
* lib/std/list.lua (foldl, foldr): Use functional.reduce instead
of deprecated functional.fold.
* lib/std.lua.in (barrel): Install _G.fold from
`std.functional.reduce`.
* specs/std_spec.yaml (barrel): Adjust.
* NEWS: Update.

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