refactor: factor away std.container.__pairs.

Object pairs iteration shouldn't rely on slow in-order traversal,
we only care about key order when printing.
* lib/std/container.lua (__pairs): Remove.  By the time this is
called, private keys are already moved into the metatable, and
the client would be using ipairs and okeys for ordered traversal.
(__tostring): Use ipairs and okeys here, because we do care about
ordering.

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