Restore cpu module visibility

[BrainBacon]

This change makes it so you can refer to CPU helper functions as you would normally from a standard rust module.

e.g.

my_shader_module::on_cpu::my_helper()

becomes

my_shader_module::my_helper()

There are still some oddities, namely that currently user types are reexported by the parser instead of marking them pub like one would normally do in a standard rust module. I may address this in a future patch.

[Sheldonfrith]

Yeah, I suppose putting that stuff under the namespace on_cpu mainly just made sense to me because they had to be generated slightly differently in the proc macro, but no need to expose those implementation details to the user, just adds confusion, so I'm fine with this change.