Replace docopt with clap

[palango]

This PR replaces docopt with clap.

Something similar was worked on in #737, but now clap includes the the derive macros that structopt had.

This also makes it possible to remove the serde dependency.

[cuviper]

I notice that the options are positioned less flexibly now -- is there a way to relax that?

e.g. life play --fps 144 fails, while life --fps 144 play works, but both worked before.

[LoganDark]

IMHO, clap may be overkill here. You could probably use something like gumdrop instead, which outperforms every other crate in the ecosystem except for getargs (which does not have derive).

I am not sure if gumdrop supports options after positional arguments (getargs does).

[palango]

I notice that the options are positioned less flexibly now -- is there a way to relax that?

Not that I'm aware of.

Do you think that this is worth changing, or should I just close the PR?

[cuviper]

Like I said in #737, I don't really have an opinion on which crate we use -- especially now that I've moved that example program under rayon-demo, so this choice doesn't affect anything we actually publish.

The code does look cleaner here, so that's nice, but the option order inflexibility is a little annoying. Maybe those can be simple positional arguments instead of sub-commands? But I don't know if that will show up well in --help.