Implement `ShaderType` for packed types

[daxpedda]

I'm hitting the issue where I want to implement ShaderType over a struct that has a [u16; 2] field, which isn't supported by Encase.

AFAIU seeing that WGSL doesn't, it makes sense. The way I make this currently work is by representing [u16; 2] as a u32 in WGSL and then unpack it into a vec2<u32>.

Is this in scope for Encase?

[daxpedda]

On that note: packed integers will probably receive more support in WGSL by things like gpuweb/gpuweb#2677.

[teoxoy]

The problem I see with implementing ShaderType for [u16; 2] right now is that we might get an extension in the future which will enable the usage of u16 types. If that happens, the [u16; 2] could be interpreted as both a packed u32 or an array of 2 u16s.

I would recommend users create newtypes for packed scalars with their own ShaderType implementation.

encase could provide helper macros that derive those implementations and/or these newtypes directly, but the question then becomes how should those types look like/what is their API?