refactor: move lambda back to std.functional.

* lib/std.lua.in (lambda): Move from here...
* lib/std/base.lua (lamba): ...and here...
* lib/std/functional.lua (lambda): ...to here.
* specs/functional_spec.yaml, specs/std_spec.yaml: Adjust
accordingly.
* lib/std/base.lua (tostring): Unroll lambda calls.
* lib/std/operator.lua ("#"): Move implementation from here...
* lib/std/base.lua (len): ...to here. Adjust all callers.
(_len): Remove.
(operator): Remove unused require statement
* lib/std/list.lua (transpose): Unroll lambda call.
* NEWS: Update.

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