Integrate dispatch2 from https://github.com/marysaka/dispatch2

Part of #77.

crates/dispatch2/Cargo.toml

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+[package]
+name = "dispatch2"
+version = "0.1.0"
+authors = ["Mary <[email protected]>"]
+license = "Apache-2.0 OR MIT"
+repository = "https://www.github.com/marysaka/dispatch2.git"
+homepage = "https://www.github.com/marysaka/dispatch2"
+description = "Bindings and wrappers for the Grand Central Dispatch (GCD)"
+keywords = ["gcd", "macOS", "iOS", "watchOS", "ipadOS"]
+categories = ["api-bindings", "development-tools::ffi", "os::macos-apis"]
+edition = "2021"
+
+exclude = [
+    ".github"
+]
+
+[dependencies]
+libc = "0.2"

crates/dispatch2/README.md

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+# dispatch2
+
+Allows interaction with the [Apple Dispatch](https://developer.apple.com/documentation/dispatch) library in a safe and unsafe way.
+
+## Usage
+
+To use `dispatch2`, add this to your `Cargo.toml`:
+
+```toml
+[dependencies]
+dispatch2 = "0.1.0"
+```
+
+## Example
+
+```rust
+use dispatch2::{Queue, QueueAttribute};
+
+fn main() {
+    let queue = Queue::new("example_queue", QueueAttribute::Serial);
+    queue.exec_async(|| println!("Hello"));
+    queue.exec_sync(|| println!("World"));
+}
+```
+
+## License
+
+dispatch2 is distributed under the terms of either the MIT license or the Apache
+License (Version 2.0), at the user's choice.
+
+See [LICENSE-APACHE](LICENSE-APACHE) and [LICENSE-MIT](LICENSE-MIT).

crates/dispatch2/TODO.md

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+# TODO
+
+- Fully document all ffi APIs
+- CI test on macOS
+- CI test on Linux using https://github.com/apple/swift-corelibs-libdispatch
+- CI test on Windows using https://github.com/apple/swift-corelibs-libdispatch
+- Safe wrapper for ``dispatch_source_*`` + ``set_target_queue/activate/suspend/resume`` for it
+- Safe wrapper for ``dispatch_data_*``
+- Safe wrapper for ``dispatch_once_f`` (is that relevent?)
+- Safe wrapper for ``dispatch_get_context/dispatch_set_context`` (quite impossible without big overhead => wrap dispatch object destructor to release the boxed value)
+- All blocks related bindings and ``dispatch_block_*`` functions with compat with ``block2`` on Apple platforms.
+- Integrate conversion from SystemTime to dispatch_time_t via dispatch_walltime and safe APIs using that.