cargo gpu clippy and check

[Firestar99]

• cargo gpu clippy allows you to run clippy with a spirv target, which can check code hidden by #[cfg(target_arch = "spirv")], that cargo clippy --all-targets does not.

• Also adds cargo gpu check, not sure how useful it'll be.

• WON'T add cargo gpu fmt since cargo fmt is fully target independent. We could use the fmt of the rust-gpu rust toolchain, but at worst that'll get you different formatting from whatever stable toolchain you're using.

• enabled by

  • custom cargo commands in No longer require crate-type = ["dylib"] on shader crates rust-gpu#477
  • proper no artifact handing in spirv-builder: when no artifact is produced, error instead of panic rust-gpu#496

[Firestar99]

Since #128 also needs to update spirv-builder, I'll wait for that one before merging this.

[nazar-pc]

Would it be possible to make the same thing usable through programmatic API? For example, when cargo clippy is running, my build script would do clippy lint on a crate too (right now I just skip the build completely for clippy).

[Firestar99]

Both enum Command and Command::run are public, so you can just construct an instance and call run? Though after #132, you'd have to depend on the full cargo-gpu and won't be able to use the minified cargo-gpu-install.
Although, you could easily replicate it's behaviour by changing the spirv_builder.cargo_cmd to "clippy" and check that the build fails with NoArtifactProduced. Maybe we should even move that to cargo-gpu-install, since it is that trivial?