bpaf_derive: do the right thing for `collect`

[pacak]

Currently this code would pass for Vec<String> but not for other collection types. Correct behavior when collect is present would be detect to "peel" the outer level from the type (BTreeSet part) and put it into collect part part while passing inner part (String) to argument.

#[test]                                                                               
fn collect_catch_set() {                     
    let input: NamedField = parse_quote! {                              
        #[bpaf(argument("FILE"), collect, catch)]     
        files: BTreeSet<String>                                                     
    };                                                                          
    let output = quote! { => TokenStream                                        
        ::bpaf::long("files").argument::<String>("FILE").collect::<BTreeSet<_>>().catch()
    };                                                                          
    assert_eq!(input.to_token_stream().to_string(), output.to_string());        
}