`SystemBuffer` requires `queue()` to be implemented

SystemBuffer now requires queue() to be implemented, instead of apply(). apply()'s default implementation now delegates to queue().

This is to ensure that a SystemBuffer used in an Observer context applies its changes. In most cases, if apply() does not change the World structurally, apply() and queue() can mutate the World directly in the same way.

If apply() does not change the World structurally, apply() should be changed to queue():

// 0.18
impl SystemBuffer for MySystemBuffer {
  fn apply(&mut self, system_meta: &SystemMeta, world: &mut World) {
    // your impl here
  }
}

// 0.19
impl SystemBuffer for MySystemBuffer {
  fn queue(&mut self, system_meta: &SystemMeta, mut world: DeferredWorld) {
    // your impl here, using a DeferredWorld instead
  }
}

If apply() does change the World structurally, implement both apply() and queue(). To queue structural changes to a DeferredWorld, add the structural changes to its command queue, accessible via world.commands().