std: use numeric keys first ordering when stringifying a table.

Closes #81.
* specs/table_spec.yaml (okeys): Specify behaviours of a new
ordered keys function.
* lib/std/container.lua (__pairs): Factor key sorting algorithm
from here...
* lib/std/base.lua (keysort): New function. ...to here.
(okeys): Use keysort to extract a sorted key list from a table.
* lib/std/table.lua (okeys): Re-export from here.
* lib/std/base.lua (render): Simplify accordingly, and always
sort keys nicely as a pleasant side-effect.
* NEWS: Update.

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