Use structopt instead of docopt

[lu-zero]

It might or might not be nicer to use. (this got prompt by docopt somehow crashing rustc nightly on power9)

[cuviper]

I think if we change this, it should be done for rayon-demo too for consistency. Direct clap is another option, since structopt is just a fancy wrapper on that.

Or since this is just a tiny example program, I think it would be fine with manual arg parsing, and get rid of the dev-dependency in rayon altogether.

(this got prompt by docopt somehow crashing rustc nightly on power9)

ref: rust-lang/rust#70041

[lu-zero]

I prefer using structopt (or gumdrop if we want to keep the build times smaller) since it makes the code more compact, not distracting from the important parts and produces informative --help that do not go out of sync easily.

I can convert rayon demo today if there is consensus for it.

[cuviper]

I can convert rayon demo today if there is consensus for it.

I don't have a strong opinion on what we use, especially since this is only for examples and demos. The fact that docopt declares itself unmaintained is reason enough to find something else. If you're willing to do the rest of the conversion, structopt should be fine.

[nikomatsakis]

For the record I'm fine with structopt :)