clap_derive: support fixed arrays as shortcut for number_of_values = N

[CreepySkeleton]

Transferred from: TeXitoi/structopt#349

It would be even better if fixed sized arrays or tuples with different value types would be supported directly:

#[derive(Clap)]
struct Args {
    #[clap(long)]
    range: [u32; N], // implies `required = true` and `number_of_values = N` 

    #[clap(long)]
    range: Option<[u32; N]>, // `number_of_values = N` 
}

[pksunkara]

Could you create a separate issue for the tuples?

[cecton]

Yeah actually having something automatic for tuples would solve my issue as I won't have to fight with Vec. (But I still believe the current impl of Vec is wrong but I didn't read your answer yet.)

[CreepySkeleton]

This change would be breaking if implemented after 3.0 is out, so triaging for 3.0

[epage]

This also ties into

• Provide data in the application domain rather than the CLI domain #2683
• clap_derive: support tuples as shortcut for number_of_values = N, different validators #1717
• Existing key-value handling which we wouldn't want to break

[epage]

I've pushed this back to design as we'd need to figure out how people can opt-out of this behavior in case a value parser wants to output one of these