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

[Zoltan Somogyi]

I have now reordered the contents of io.m as well.
The only part that I think remains to be improved is that
at the moment, almost all the foreign_decls and foreign_codes
are in a big bunch at the start of the implementation section.
These should be broken up into chunks that correspond to
the various sections of the file, and each chunk moved to
the front of its chunk. However, three quarters of that job
is best done by people who understand C#, Java and Erlang
better than I do, so I left it for them.

Zoltan.