list: exchange parameter order for list.cons.

Closes #72.
* specs/list_spec.yaml (cons): Modernize specifications.
Specify deprecation warning behaviour when calling cons with
arguments in legacy order.
* lib/std/list.lua (cons): If arguments appear to be in the
wrong order, issue a deprecation warning and rewrite them into
the correct order.
* NEWS: Update.

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