Derive Macro hugr_cli::Parser

#[derive(Parser)]
{
    // Attributes available to this derive:
    #[clap]
    #[structopt]
    #[command]
    #[arg]
    #[group]
}

We reexport some clap types that are used in the public API. Generates the Parser implementation.

This is far less verbose than defining the clap::Command struct manually, receiving an instance of clap::ArgMatches from conducting parsing, and then implementing a conversion code to instantiate an instance of the user context struct.