[m-dev.] more things to move from io.m.

[Zoltan Somogyi]

io.m is now down to 6700 lines (from more than 13k originally).
Nevertheless, there are more things we can move from it to other places.

One is the predicates flush_output_2 and flush_binary_output_2.
We could move these to either io.stream_ops, io.primitives_write,
or to a new submodule that writes out non-primitive items. I think
stream_ops is the right place.

Another is the implementations of read_word, read_line, read_file
and their variants. I think these can be moved to a new submodule
(leaving the top-level predicates in io.m); the main question is what this
new submodule should be named. These are the only non-primitive things
io.m reads that take up any space in io.m itself. Reading terms is done by
mercury_term_parser.m; reading bitmaps is now done by bitmap.m.
How about io.read_text.m, or io.text_read.m to fit in with io.primitives_read.m?

I just checked, and there are three uses of the input_stream_foldl
predicates in the Mercury implementation. All three would be simpler
and probably faster if they were replaced by code that read in the relevant
file as a string and operated on that. So I have no objection to Julien's
proposal a while ago to mark them all as obsolete. However, I do think
that before we do so, we should add a version of list.foldl that does
two-level iteration, to reduce the max recursion depth to tolerable levels
in grades that do not have tail recursion (mainly debug and profiling grades).
And that throws up the obvious $64k question: exactly which subset
of the gazillion foldl predicates should get such a version? I envision
those versions would have an extra input arg specifying the chunk size.

I also intend to remove from io.m the internal-only with_input_stream
predicates, by making their callers use explicit streams.

Opinions? Objections?

Zoltan.