Add bindings for spirv-opt

[Jake-Shadle]

We will be relying on spirv-opt for actually making the generated spirv code go brrrrr, but right now there are no rust bindings for it, so we can't call it from Rust like we need to.

There are a couple of complications to this.

1. The code is all C++, and there's no C API
2. CMake, because of course

For the first point, the actual API for running the optimizer seems to be fairly trivial, so I see 2 paths forward for this.

1. Write a small C wrapper around the C++ API and write manual bindings for it
2. Try out cxx though my guess right now is that it is overkill

As for CMake, just as with our PhysX bindings, we will obviously just build it with cc so that downstream users won't hate us.

I'm putting this issue here for now, but I am thinking that this would probably be its own repo which contains the bindings/wrapper crates in it, and it would initially just be for the spirv-opt part of spirv-tools, but obviously could add the other tools as needed.

[Jake-Shadle]

I guess we may also need spirv-val?

[khyperia]

Maybe - it depends on some design decisions we make.

Right now, it's used purely as a debugging development tool, and end users are not expected to run it. So, it'd be fine to just expect developers to have spirv-val installed, and not put in the effort to make bindings. However, there are some design decisions that might lead to end users running spirv-val, in which case, bindings.

(I should make an issue on the exact details of those decisions...)