Vendor dependencies for 0.3.0 release

vendor/async-executor/README.md

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+# async-executor
+
+[![Build](https://github.com/smol-rs/async-executor/actions/workflows/ci.yml/badge.svg)](
+https://github.com/smol-rs/async-executor/actions)
+[![License](https://img.shields.io/badge/license-Apache--2.0_OR_MIT-blue.svg)](
+https://github.com/smol-rs/async-executor)
+[![Cargo](https://img.shields.io/crates/v/async-executor.svg)](
+https://crates.io/crates/async-executor)
+[![Documentation](https://docs.rs/async-executor/badge.svg)](
+https://docs.rs/async-executor)
+
+Async executors.
+
+This crate provides two reference executors that trade performance for
+functionality. They should be considered reference executors that are "good
+enough" for most use cases. For more specialized use cases, consider writing
+your own executor on top of [`async-task`].
+
+[`async-task`]: https://crates.io/crates/async-task
+
+## Examples
+
+```rust
+use async_executor::Executor;
+use futures_lite::future;
+
+// Create a new executor.
+let ex = Executor::new();
+
+// Spawn a task.
+let task = ex.spawn(async {
+    println!("Hello world");
+});
+
+// Run the executor until the task completes.
+future::block_on(ex.run(task));
+```
+
+## License
+
+Licensed under either of
+
+ * Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
+ * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+
+at your option.
+
+#### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.