naga-types

Naga-types contains some types used by both naga and wgpu. Naga may be an optional dependency of wgpu in the future, so these can't live in naga. Additionally, naga is a mostly independent crate, so it cannot depend on wgpu-types. For this reason, the types must live in a naga-specific crate but not naga itself. Naga-types serves that purpose.