list: deprecate index_key and index_value.

* specs/list_spec.yaml (index_key, index_value): Check that the
functions issue a depraction warning on first use.
* lib/std/list.lua (index_key, index_value): Add deprecation
warning with base.deprecate.
* NEWS (Incompatible changes): Update.

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