refactor: use std.ipairs and std.pairs everywhere internally.

* lib/std/container.lua, lib/std/functional.lua, lib/std/io.lua,
lib/std/list.lua, lib/std/optparse.lua, lib/std/package.lua,
lib/std/set.lua, lib/std/string.lua, lib/std/table.lua,
lib/std/tree.lua, lib/std/vector.lua: Import and use `base.ipairs`
and `base.pairs` everywhere
* NEWS: Update.

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