legion::world::World::get_component_mut_or_default() ?

Hi,

Would it be possible to have `legion::world::World::get_component_mut_or_default()` to be able to create or get, then modify a component while minimizing code and accesses ?
This would be to avoid code like:
```rust
if let Some(mut component) = self.world.get_component_mut(entity) {
return component;
}
self.world.add_component(entity, C::default());
self.world.get_component_mut(entity).unwrap()

```
(which doesn't even compile)