Add utility to run compute shaders, and add tests for it

[almarklein]

• Add compute_with_buffers() utility function.
• Add tests for it. This'll be the first time we will be actually running wgpu on CI :)

After this makes it into a release, python-shader could use it to better validate the compiler.

At this this point this is blocked by #47: the added test can't run on CI. If there is no quick solution for it, I propose we disable the test on CI, so that we can at least run it locally.

[Korijn]

For now, let's just add an option to skip the tests and merge so we can continue with development, and use #47 to track the problem.

[almarklein]

Ready for final checkup. Changes since last review:

• The new module was moved to utils subpackage.
• We already had a utils module, so I renamed that to _coreutils.
• No more numpy, but ctypes arrays. An extra nice thing about this is that output_arrays must now be specified with just a ctypes array type, which includes the length :)
• A wrapper that works with numpy arrays is easily made, because numpy operates well with ctypes. (but that wrapper is not in our scope).
• The tests are skipped if wgpu cannot get a device (this is checked once in a test run).

[Korijn]

I'm not too familiar with ctypes but it looks good!