Allow full paths in `world!`'s component field

This allows the use of full types when generating component code, such as
```rust
world! {
    components Position<f64>;
}
```
or
```rust
world! {
    components some::crate::Position;
}
```
while retaining the same general usage syntax as before. I haven't considered collisions, though I believe potential error messages would be quite clear.