We should figure out how to use Naga's `"span"` feature (for `example-runner-wgpu`).

[eddyb]

AFAICT, if we set features = ["span"] for naga in examples/runners/wgpu/Cargo.toml, we'll get more information on e.g. validation failures (which would've been nice this entire time, alas I didn't realize).

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EDIT: nevermind, I got confused, span is already enabled by default, it just doesn't seem to do anything useful for SPIR-V input into Naga.