Cleanup classes namespace and repr

[almarklein]

The wgpu.base module that defined the classes is technically public API, but that module contains a lot of stuff that we need to define the classes. This PR cleans that up. It's a bit nit-picky but hey, look how tidy things look!

• Renames "base.py" to "_classes.py".
• Creates "classes.py", a public namespace that contains just the classes, similar to how we have wgpu.enums and wgpu.flags.
• The repr of the classes lists the public namespace where the object is loaded from, omitting the private module (e.g. <wgpu.GPUDevice ..> or <wgpu.backends.wgpu_native.GPUDevice ..>).