Make TypeInner::scalar_width return the width in bytes

[atlv24]

TypeInner::scalar_width currently returns the scalar width in bits. This is counter-intuitive, when the width is stored in bytes and assumed to be bytes everywhere else. If the current functionality is to be maintained, it should be renamed to scalar_width_bits and documented.

https://github.com/gfx-rs/wgpu/blob/trunk/naga/src/proc/mod.rs#L267