nak: Shader info improvements

This is prep work for using NAK in non-NIR contexts such as unit testing but it's a good improvement in its own right. The idea is to plumb the few remaining bits of shader info through NAK's ShaderInfo struct rather than going straight from NIR to nak_shader_info. This means less poking at NIR outside of nak_from_nir.rs and a more straightforward compile flow.