[Proof of concept] derive specs system

[azriel91]

Relates to the RFC in issue #498. This allows you to do this:

#[specs_system(SysA)]
fn sys_a(mut pos: WriteStorage<Pos>, vel: ReadStorage<Vel>) {
    for (pos, vel) in (&mut pos, &vel).join() {
        pos.0 += vel.0;
    }
}

#[specs_system(SysPrint)]
fn sys_print(entities: Entities, pos: ReadStorage<Pos>, vel: ReadStorage<Vel>) {
    for (e, pos, vel) in (&entities, &pos, &vel).join() {
        println!("{:?}: Pos: {:?}   Vel: {:?}", e, pos, vel);
    }
}

// ...
let mut dispatcher = DispatcherBuilder::new()
    .with(SysA, "sys_a", &[])
    .with(SysPrint, "sys_print", &["sys_a"])
    .build();

and running this gives:

$ cargo run --example system_derive_with_components
    Finished dev [unoptimized + debuginfo] target(s) in 0.36s
     Running `target\debug\examples\system_derive_with_components.exe`
Entity(0, Generation(1)): Pos: Pos(2.0)   Vel: Vel(2.0)
Entity(1, Generation(1)): Pos: Pos(5.6)   Vel: Vel(4.0)
Entity(2, Generation(1)): Pos: Pos(6.9)   Vel: Vel(1.5)
Entity(0, Generation(1)): Pos: Pos(4.0)   Vel: Vel(2.0)
Entity(1, Generation(1)): Pos: Pos(9.6)   Vel: Vel(4.0)
Entity(2, Generation(1)): Pos: Pos(8.4)   Vel: Vel(1.5)

---

This change is 

[azriel91]

failed on stable, but I think it would work when 1.30 is out ^{[1, 2, 3]}. That means using this feature would bump the minimum rust version

[azriel91]

From Rust #beginners:

dyn r#CryZe:

ref is pseudo deprecated
use default match bindings instead
match &mut ty

ᚮscottmcmᚭ:

and +1 to

let segments = match &mut ty {
    Type::Path(TypePath {
        path: Path {
            segments, ..    // <-- no more silliness
        },

[torkleyy]

This should be moved to shred.

[torkleyy]

Closing since it's outside of Specs' scope and belongs to shred.