Switch from structopt to clap in examples

[johanhelsing]

Structopt is in maintenance mode.

[johanhelsing]

Oops, this changes behavior slightly. This doesn't work anymore

cargo run --example box_game_p2p -- --local-port 7000 --players localhost 127.0.0.1:7001

this does:

cargo run --example box_game_p2p -- --local-port 7001 --players 127.0.0.1:7000 --players localhost

...but I'll see if I can get the behavior to stay the same with clap

EDIT: done :)

[gschup]

good to know structopt is no longer a thing. Thanks!