Change tests/standalone so that a tool regenerates its sources

[sivadeilra]

This fixes a problem relating to the build script of the crates/tests/standalone crate. This build script runs bindgen on a variety of inputs and generates source code.

The problem is that the build script modifies source code during a build script, and does so unconditionally. Because this process is relatively slow, these filesystem writes sometimes interfere with Git operations, because Git sees the intermediate (partially modified) state of these source files. This has been a big hassle for me in running Git operations. It also interferes with Rust Analyzer, and causes a lot of wasted resources when editing the repo with RA.

The solution is to move this codegen step to a separate tool, crates/tools/standalone. This works similarly to existing tools, like windows and sys. The only difference is that this follows the new pattern of directly linking to windows_bindgen instead of running cargo run recursively. This is a lot more efficient, especially if you run cargo run --release ....