Vendor dependencies

Let's see how I like this workflow.

vendor/rustix/CONTRIBUTING.md

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+# Contributing to rustix
+
+Rustix is a [Bytecode Alliance] project. It follows the Bytecode Alliance's
+[Code of Conduct] and [Organizational Code of Conduct].
+
+## Testing
+
+To keep compile times low, most features in rustix's API are behind cargo
+features. A special feature, `all-apis` enables all APIs, which is useful
+for testing.
+
+```
+cargo test --features=all-apis
+```
+
+And, rustix has two backends, linux_raw and libc, and only one is used in
+any given build. To test with the libc backend explicitly, additionally
+enable the `use-libc` feature:
+
+```
+cargo test --features=all-apis,use-libc
+```
+
+Beyond that, rustix's CI tests many targets and configurations. Asking for
+help is always welcome, and it's especially encouraged when the issue is
+getting all the `cfg`s lined up to get everything compiling on all the
+configurations on CI.