Vendor dependencies

Let's see how I like this workflow.

vendor/rustix/COPYRIGHT

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+Short version for non-lawyers:
+
+`rustix` is triple-licensed under Apache 2.0 with the LLVM Exception,
+Apache 2.0, and MIT terms.
+
+
+Longer version:
+
+Copyrights in the `rustix` project are retained by their contributors.
+No copyright assignment is required to contribute to the `rustix`
+project.
+
+Some files include code derived from Rust's `libstd`; see the comments in
+the code for details.
+
+Except as otherwise noted (below and/or in individual files), `rustix`
+is licensed under:
+
+ - the Apache License, Version 2.0, with the LLVM Exception
+   <LICENSE-Apache-2.0_WITH_LLVM-exception> or
+   <http://llvm.org/foundation/relicensing/LICENSE.txt>
+ - the Apache License, Version 2.0
+   <LICENSE-APACHE> or
+   <http://www.apache.org/licenses/LICENSE-2.0>,
+ - or the MIT license
+   <LICENSE-MIT> or
+   <http://opensource.org/licenses/MIT>,
+
+at your option.