Support more attributes

[fasterthanlime]

@epage suggested this on Reddit:

Instead of free-form attributes, what if attributes were const-expressions that evaluated to a value that gets stored? It seems like those could be instropected like anything else in facet.

use facet_args::prelude::*;

#[derive(Facet)]
struct Args {
    #[facet(Positional)]
    path: String,

    #[facet(Named, Short('v'))]
    verbose: bool,

    #[facet(Named, Short('j'))]
    concurrency: usize,
}
Maybe you could event do a hack like clap's where Name = Value gets treated as Name(Value)

use facet_args::prelude::*;

#[derive(Facet)]
struct Args {
    #[facet(Positional)]
    path: String,

    #[facet(Named, Short = 'v')]
    verbose: bool,

    #[facet(Named, Short = 'j')]
    concurrency: usize,
}

---

I'm not sure exactly how this would work, but I'm willing to try.

[fasterthanlime]

In the latest (shipping as we speak) release, at least, the "arbitrary attribute" thing is less heinous — it's not the debug repr of the TokenTree anymore, but merely the "tokens_to_string" method or something, which looks like slightly funny Rust and is very parsable, imho.

[fasterthanlime]

I think we're well on our way to do that.