Add structopt

[Urhengulas]

This PR introduces sctructopt for handling cli-params.

• I tried to use argument names and descriptions like GNU ld
• using structopt enables us to drop fn get_output_path(...) and dramatically shorten fn get_linker_scripts(...)

[Urhengulas]

This PR is blocked by the fact that we need to accept way more args than we are actually using here. In best case we can just ignore them, but I couldn't figure out how to do that.

I've tried clap::AppSettings::TrailingVarArg, but this didn't work so far, since we also need to accept arguments in the middle, and not only at the end.

[Urhengulas]

clap-rs/clap#1404 would be helpful for solving our blocker.

[Urhengulas]

One approach I though might be useful is to "pre-filter" the arguments (std::env::args()) and then parse them with StructOpt::from_iter(...). A sketch is appended.

More generally some lightweight parser to extract specific args from the cli-args and ignore the rest, would be very useful for our kind of cli (being a cli-wrapper around another cli)!

pre-filter example

use std::{env, path::PathBuf};

use structopt::StructOpt;

#[derive(Debug, StructOpt)]
struct Opt {
    #[structopt(short = "-L", long, parse(from_os_str))]
    library_path: Vec<PathBuf>,

    #[structopt(short, long, parse(from_os_str))]
    output: PathBuf,

    #[structopt(short = "T", long)]
    script: Vec<String>,
}

fn main() {
    let app = Opt::clap().p.opts;

    // get long arguments, e.g. `--output`, `--script`
    let mut long = app.iter().filter_map(|opt| opt.s.long).collect::<Vec<_>>();
    long.extend(["help", "version"].iter());

    // get short arguments, e.g. `-o`, `-T`
    let mut short = app.iter().filter_map(|x| x.s.short).collect::<Vec<_>>();
    short.extend(['H', 'V'].iter());

    dbg!(long, short);

    // filter one element args
    let short_args = env::args()
        // check if `x` starts with '-' and 2nd char is in `short`
        .filter(|x| unimplemented!())
        .collect::<Vec<_>>();
    // filter two element args
    let long_args: Vec<&[String]> = unimplemented!();

    // merge args
    let args: Vec<String> = unimplemented!(); // shorts_args + long_args

    let opt: Opt = Opt::from_iter(args);
    dbg!(opt);
}