add explicit `#[gpu_repr(storage)]` attribute to `#[derive(GpuLayout)]`

[RayMarch]

Right now, the WGSL storage layout is the default layout applied to structs when #[derive(GpuLayout)] is used.
For consistency, and to allow maybe adding a #[gpu_repr(uniform)] in the future, the following changes need to be made:

• rename TypeLayoutRules::Wgsl to TypeLayoutRules::Storage
• rename TypeLayoutRules to shame::mem::Repr
• add a shame::mem::Repr argument to conversions from the ir::Type family to TypeLayout
• add a #[gpu_repr(storage)] attribute similar to the existing #[gpu_repr(packed)] to #[derive(GpuLayout)]