operator: more RSI-reducing operator function name changes.

* specs/operator_spec.yaml (cons, length): Remove - just pass
`table.pack` or `table.len` instead, resp.
(["and"], ["or"], ["not"]): Rename these...
(conj, disj, neg): ...to these.
* lib/std/operator.lua: Likewise.
Give full and proper LDocs for each function.
* lib/std/functional.lua (M.op): Update deprecation redirections.
* NEWS: Update.

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