Implement casting and basic arithmetic

[almarklein]

• [x] Refactor tests to make it easy to run shaders where we can, validate where we can, and at the least compare the hashes.
• [x] Automatically add SpirV capabilities as "special types" (e.g. int16, f64) are used.
• [x] Implement casting of scalars from one type into another.
• [x] Implement casting of vectors from one type into another.
• [x] Fix notion of unsigned vs signed integers.
• [x] Implement some basic arithmetic.
• [x] Loads of tests.
• [ ] More tests to reach 100% coverage for these code paths --> for another PR.

To clarify:

• In all tests runs, it is validated that the bytecode and spirv of (nearly) all shaders in the tests are the same as the reference (using hashes).
• Where possible (local and on one CI run), spirv-val is used to validate the spirv of all shaders.
• Where possible (local only, because we cannot use wgpu on CI), some (compute) shaders are run and their result validated.

[Korijn]

Why do you need wgpu here actually? It's cool that it can be used so easily here by the way :)

[almarklein]

Why do you need wgpu here actually?

So that we can actually run the shaders in the tests to validate that they produce the expected result. I'll update the first comment to better explain when/where what is actually tested.

It's cool that it can be used so easily here by the way :)

Yeah, I'm happy with that compute_with_buffers() :)

[Korijn]

I'll be honest, I don't have the time I would need to review this in detail available. I looked at the tests and they seem to indicate success. :) LGTM