`set_executor` replaced `ExecutorKind`

ExecutorKind has been removed. Schedules are now configured by passing an executor instance directly via Schedule::set_executor.

• Schedule::set_executor_kind has been removed. Use Schedule::set_executor instead.
• Schedule::get_executor_kind has been removed. There is no replacement; executors are no longer identified by an enum variant.
• SystemExecutor::kind has been removed from the trait.
• SystemExecutor is now a public trait. You can implement it to provide a fully custom executor.
• SystemSchedule::systems is now pub.

// 0.18
use bevy::ecs::schedule::ExecutorKind;

schedule.set_executor_kind(ExecutorKind::SingleThreaded);
schedule.set_executor_kind(ExecutorKind::MultiThreaded);
schedule.set_executor_kind(ExecutorKind::default());

// 0.19
use bevy::ecs::schedule::{SingleThreadedExecutor, MultiThreadedExecutor, default_executor};

schedule.set_executor(SingleThreadedExecutor::new());
schedule.set_executor(MultiThreadedExecutor::new());
schedule.set_executor(default_executor());