[m-rev.] for post-commit review: reorder the interface of io.m

[Zoltan Somogyi]

I am not attaching the diff, because it is a mess. Instead, I am asking
any volunteer reviewer to read the updated interface section of io.m.

I am committing the change because while the new order may not be
perfect, it is definitely much better than what went before.

I intend to modify the implementation only after there is a consensus
on the interface.

Zoltan.