refactor: move `table.ripairs` to `lua.ripairs`.

* specs/table_spec.yaml (ripairs): Specify deprecation warning
on first use.
* specs/lua_spec.yaml (ripairs): Specify all behaviours for
ripairs.
* lib/std/base.lua (ripairs): Shared core functionality for
ripairs, respecting `__ipairs` metamethod even on Lua 5.1.
* lib/std/table.lua (ripairs): Use it, with a deprecation
warning on first use.
* lib/std/lua.lua (ripairs): Re-export it from here with full
argchecks.
* NEWS: Update.

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