list: index_value and index_key return raw tables.

* specs/list_spec.yaml (index_key, index_value): Specify proper
behaviours.
* lib/std/list.lua (index_key, index_value): A non-contiguous
set of valid results cannot be represented as a std.list object,
so return a raw table.

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