Remove --no-spirt and the old structurizer.

[eddyb]

• completes the plan from Minimally integrate SPIR-T (opt-in via RUSTGPU_CODEGEN_ARGS=--spirt). #940
• closes Consider removing --no-spirt (and outdated codepaths) in Rust-GPU 0.8. #1045
  • blocked on its blockers (the Naga loop issue)
• completely removes the code added in New structurizer: now with ∞% more φ! #287
  • i.e. my first non-trivial Rust-GPU contribution, is now replaced by SPIR-T

The test commits should be reviewed separately, as I tried to not mix the different kinds of changes to them.

---

The --no-spirt documentation and changelog have this note in them:

Note: if you were using --no-spirt to work around naga issue #1977
(valid loops causing The 'break' is used outside of a 'loop' or 'switch' context),
you may be able to cargo update -p naga to update to a fixed naga version
(0.11.1 for wgpu 0.15, 0.12.1 for wgpu 0.16, and any later versions).

Hopefully that's detailed enough, but I expect the release notes to expand on it even more.
(It's looking like Rust-GPU 0.8 will mostly only be this change, which should make it even harder to miss)

[fu5ha]

🎉 I didn't review super thoroughly but it all looks reasonable to me and super exciting!! Great work on spirt and exciting to see it beginning to bear more and more fruit :)

[eddyb]

Did another skim myself to confirm that none of the test changes broke anything.