Unnecessary error in #[system] validation

Was playing with legion (3.1.0) and trying to copy various examples of systems, but kept running into the "simple systems cannot contain component references" error.
https://github.com/amethyst/legion/blob/139a2ad953f119a470934abafdb50ee9c43beb00/codegen/src/lib.rs#L551

I eventually realized that I needed to import the SubWorld rather than qualifying it. I was writing this:
```rust
#[system]
#[read_component(usize)]
#[write_component(bool)]
fn run_query(world: &mut world::SubWorld) {
let mut query = <(&usize, &mut bool)>::query();
for (a, b) in query.iter_mut(world) {
println!("{} {}", a, b);
}
}
```

which is an example from the docs with the `world::` added before SubWorld, and then I got the error above. Pretty new to rust so I expected this to be equivalent to importing and then using it.

Importing it is a fine workaround for me but I thought I'd mention it case it helps someone! Thanks!

Edit: looked a bit more at the code and maybe it's just a matter of adding the "world::SubWorld" case here? I was importing legion as `use legion::*` as it says in the docs in a few places, which made my editor automatically suggest the qualification relative to "world" rather than "legion", I guess. Either that or expose SubWorld from legion::* I guess (didn't seem like it was).
https://github.com/amethyst/legion/blob/139a2ad953f119a470934abafdb50ee9c43beb00/codegen/src/lib.rs#L359