refactor(cli): migrate from structopt to clap-derive

[tranzystorekk]

structopt is quite old and unmaintained, and porting its code to pure clap-derive is straightforward for this codebase.

Hope this proves useful, but here are some additional notes:

• I added value names to options that seem more appropriate to me
• I wanted to use the new value_parser mechanism provided by clap, but that required adjusting the type of words to u32 and casting it later to usize, as clap doesn't support *size types here.

[max-niederman]

Awesome, looks great!