This PR changes how storage classes where defined to their own module, refactors the macro to allow you to define multiple storage classes in a single macro invocation and more importantly allows you to add documentation to the storage classes. With the refactor I also changed the DSL you use to define storage so that's a more descriptive, and hopefully more intuitive, for example here is now what the definition of the Input and Output storage classes look like.

storage_class! {
    /// Input from pipeline.
    ///
    /// Visible across all functions in the current invocation. Variables
    /// declared with this storage class are read-only, and must not
    /// have initializers.
    #[spirv(input)] storage_class Input;

    /// Output to pipeline.
    ///
    /// Visible across all functions in the current invocation.
    #[spirv(output)] writeable storage_class Output;
}