refactor: make RustScan additionally a library

[PsypherPunk]

• add lib.rs (permitted as per Packages and Crates).
• refactor main.rs to reference the library.
• update module references as required.

relates #497

By way of example, configuring a new binary project which references this crate:

[dependencies]
async-std = "^1.7"
rustscan = { path = "/home/rust/git/RustScan" }

…allows rustscan to be referenced as a library:

use async_std::task::block_on;
use std::{net::IpAddr, time::Duration};

use rustscan::input::{PortRange, ScanOrder};
use rustscan::port_strategy::PortStrategy;
use rustscan::scanner::Scanner;

fn main() {
    let addrs = vec!["127.0.0.1".parse::<IpAddr>().unwrap()];
    let range = PortRange {
        start: 1,
        end: 1_000,
    };
    let strategy = PortStrategy::pick(&Some(range), None, ScanOrder::Random);
    let scanner = Scanner::new(
        &addrs,
        10,
        Duration::from_millis(100),
        1,
        true,
        strategy,
        true,
        vec![9000],
    );

    let scan_result = block_on(scanner.run());

    println!("{:?}", scan_result);
}

[PsypherPunk]

Also, tests pass except that as this is now a library, doc-tests are running and failing.

[bee-san]

Wow this is cool!!!! A long awaited feature. I think some docs on how to use the library would be great too 🥳

[PsypherPunk]

@bee-san, I've fixed the failing parts:

• fixed the doc-tests in src/benchmark/mod.rs.
• changed the indentation in src/scripts/mod.rs as this was being interpreted as code—this might need a review as I'm not sure if it looks as intended in the resulting doc. output.
• I've marked a couple in src/scanner/mod.rs as compile_fail; I don't think there's a particularly easy way to instantiate a Scanner for the examples—I suppose things like this are going to be raised if this is to be used as a library; should there be a sensible Default for Scanner?

Re. the "docs on how to use the library", I'll see what I can do…

[bee-san]

Wow! Thank you so much!!