build(rust): vendor rnix parser crate

third_party/crates/rowan-0.16.1/README.md

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+# Rowan
+
+[![Crates.io](https://img.shields.io/crates/v/rowan.svg)](https://crates.io/crates/rowan)
+[![Crates.io](https://img.shields.io/crates/d/rowan.svg)](https://crates.io/crates/rowan)
+
+Rowan is a library for lossless syntax trees, inspired in part by
+Swift's [libsyntax](https://github.com/apple/swift/tree/5e2c815edfd758f9b1309ce07bfc01c4bc20ec23/lib/Syntax).
+
+A conceptual overview is available in the [rust-analyzer repo](https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/dev/syntax.md).
+
+See `examples/s_expressions` for a tutorial, and [rust-analyzer](https://github.com/rust-analyzer/rust-analyzer/) for real-world usage.
+
+## Testing
+
+This crate is primarily tested by various integration tests in rust-analyzer.
+
+## License
+
+Rowan is primarily distributed under the terms of both the MIT
+license and the Apache License (Version 2.0).
+
+See LICENSE-APACHE and LICENSE-MIT for details.