lua: support __ipairs and __pairs metamethods on Lua 5.1.

* specs/lua_spec.yaml (ipairs, pairs): Specify portable behaviour
for new functions.
* lib/std/lua.lua (ipairs, pairs): New functions that support
__ipairs and __pairs metamethods, even on Lua 5.1.
(ielems, elems): Improve LDocs and argchecks.
* specs/lua_spec.yaml (elems, ielems): Adjust error message specs.
* NEWS: Update.

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