glam-infication of spirv-std

[Firestar99]

Requires #394 (due to conflicts)

see #393

How are people feeling on replacing all the trivial impl Vector<f32, 4> with a plain glam::Vec4?

pro:

• clearer to read
• functions returning V: Vector don't need any explicit declarations that it's a Vec2, in case of ambiguity:
  

  rust-gpu/crates/spirv-std/src/float.rs

  Line 225 in fb42c2b

  pub fn u16x2_to_vec2_unorm<V: Vector<f32, 2>>(int: u32) -> V {

contra:

• Both Vec3 and Vec3A impl Vector<f32, 3>, so you can't use them interchangeably anymore. We expect most people to use Vec3, and conversion is a trivial From::from.

Cases where we are generic on the vector element type, or element count, or both should remain as generic, eg:

rust-gpu/crates/spirv-std/src/arch/subgroup.rs

Line 336 in fb42c2b

pub fn subgroup_broadcast_first<T: VectorOrScalar>(value: T) -> T {

closes #393

[schell]

Great!