Add arrays of buffer descriptors

[Firestar99]

This PR adds support for arrays of buffer descriptors, and represents them like this in rust-gpu:

#[spirv(fragment)]
pub fn main(
    #[spirv(descriptor_set = 0, binding = 4, storage_buffer)] sized_buffer: &mut RuntimeArray<SizedBuffer>,
    #[spirv(descriptor_set = 0, binding = 5, storage_buffer)] unsized_buffer: &mut RuntimeArray<[u32]>,
) {}

This is a breaking change to how RuntimeArray operates. However, on all previous versions using it in such a way would result in a warning similarly to below. So I'm hoping this should have discouraged anyone from using it like that, and this change does not actually break anyone' code.

  warning: use &[T] instead of &RuntimeArray<T>
   --> example-shader/src/image_shader.rs:8:78
    |
  8 |     #[spirv(descriptor_set = 0, binding = 0, storage_buffer)] _sized_buffer: &mut RuntimeArray<SizedBuffer>,
    |                                                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  warning: `example-shader` (lib) generated 1 warning

But while trying to record that warning, I've noticed that even with a #![deny(warnings)] in the lib.rs of my shader crate, it would still only emit as a warning, thus when building via a build script not even show up to the end user.

Background

Currently we can represent the following glsl buffer declarations in rust-gpu:

layout(set = 0, binding = 0) buffer SizedBuffer {
    Mat3 model_matrix;
    uint id;
} sizedBuffer;

layout(set = 0, binding = 1) buffer UnsizedBuffer {
    uint data[];
} unsizedBuffer;

layout(set = 0, binding = 2) uniform image2D singleImage;
layout(set = 0, binding = 3) uniform image2D manyImages[];

pub struct SizedBuffer {
    pub model_matrix: glam::Mat3,
    pub id: u32,
}

#[spirv(fragment)]
pub fn main(
    #[spirv(descriptor_set = 0, binding = 0, storage_buffer)] sized_buffer: SizedBuffer,
    #[spirv(descriptor_set = 0, binding = 1, storage_buffer)] unsized_buffer: &mut [u32],
    #[spirv(descriptor_set = 0, binding = 2)] single_image: &Image!(2D, type=f32, sampled),
    #[spirv(descriptor_set = 0, binding = 3)] many_images: &RuntimeArray<Image!(2D, type=f32, sampled)>,
) {}

While we can have arrays of image or sampler descriptors, we cannot represent arrays of buffer descriptors, like we can in glsl. The proposed syntax for rust-gpu at the very top of this PR equals the following in glsl:

layout(set = 0, binding = 4) buffer SizedBuffer {
   Mat3 model_matrix;
   uint id;
} manySizedBuffers[];

layout(set = 0, binding = 5) buffer UnsizedBuffer {
   uint data[];
} manyUnsizedBuffers[];

[Firestar99]

I cannot figure out why the specializer is upset about my changes (or what exactly it does in the first place). For now I've just disabled the error reporting, and the spirv emitted looks fine. So I assume this is a bug / missing feature in the specializer rather than my PR?
Any help would be appreciated :D