Add Naga variant to ShaderSource

[atadier]

Connections
Implemention of #2788

Description
Adds a new Naga(Cell<naga::Module>) variant to ShaderSource. The module has to be wrapped in std::cell::Cell because naga::Module does not implement Copy.

Testing
cargo check --features naga passed on native target and WebAssembly.

[jimblandy]

If we use Cell in wgpu::ShaderSource, then that makes it not implement Sync, which can be a pain. I think it should be fine if we simply pass in the naga::Module by value:

• give ShaderSource a variant Naga(naga::Module), that owns the module,
• change create_shader_module to simply take the ShaderModuleDescriptor by value, not by reference, and
• plumb that through wgpu::Context::device_create_shader_module.

Then the direct back end can just move the naga::Module into the wgpu_hal::ShaderInput it creates.

This is a breaking API change, but now's the time to make such changes. I've pushed changes to pass ShaderModuleDescriptor by value here, in case you want to rebase your change on that: https://github.com/jimblandy/wgpu/tree/pass-ShaderModuleDescriptor-by-value
It's a big patch, but it's mechanical.

[atadier]

Had to force push to fix a git manipulation. The change should now be rebased correctly.

[cwfitzgerald]

Cool beans