Consider marking streams as `MustUse`.

It is a bit of a smell to have unused timely dataflow streams. They are not harmful (data terminate at operators that are not used, rather than eagerly cloned for an unused stream), but it is odd to create one and not use it. We could mark them as MustUse which will at least warn if streams are created and then dropped without being used.

The main downside seems to be that it would introduce warning noise to all timely projects, as e.g. the pattern

mystream
    .do_stuff()
    .probe_with(&mut probe);

ends with a stream (probe_with outputs its input stream, after the probe has been updated). Perhaps we'd want a .cap() method to make it easy to remove the error. Putting a let _ = at the beginning of each of these pipelines might be annoying. Anyhow, food for thought.

cc: @benesch

Start by reviewing the stream-producing APIs and the probe_with pipeline pattern described in the issue. Determine the project-wide impact of marking streams MustUse and how unused returned streams should be handled. Done requires an agreed design that avoids warning noise while providing the proposed unused-stream warning.