Add Bastion::spawn method

[Stupremee]

This Pull Request adds a method to quickly create a children with a given action without having to
use the closure in Bastion::children.

Checklist

• tests are passing with cargo test.
• tests and/or benchmarks are included
• documentation is changed or added
• commit message is clear

Example

use bastion::prelude::*;

fn main() {
    Bastion::init();
    
    let children_ref: ChildrenRef = Bastion::spawn(|ctx: BastionContext| {
        async move {
            // ...
            Ok(())
        }
    }).expect("Couldn't create the children group.");
    Bastion::start();
    Bastion::stop();
    Bastion::block_until_stopped();
 }

This change was requested in #116

[vertexclique]

First, welcome to the Bastion 🏰 !
Second, looks good