[WIP] add #[spirv(typed_buffer)] for explicit SpirvType::InterfaceBlocks.

[eddyb]

Opened as draft because it's somewhat incomplete, but I want it out of the way for now.

This PR adds a TypedBuffer<T> "handle" type to spirv-std (comparable to Image types in some respects).
While not a perfect representation of the Vulkan/SPIR-V buffer model, we can't do much better for now.

With TypedBuffer<T>, we can now redefine the existing usage patterns:

#[spirv(fragment)]
fn main_fs(
    #[spirv(uniform,        descriptor_set = 0, binding = 0)]
    uniform_buffer: &MyConstData,
    #[spirv(storage_buffer, descriptor_set = 0, binding = 1)]
    storage_buffer: &mut [u32],
) {...}

as sugar¹ for:
_{¹ (which we will continue to support for the foreseeable future, presumably)}

#[spirv(fragment)]
fn main_fs(
    #[spirv(uniform,        descriptor_set = 0, binding = 0)]
    uniform_buffer: &TypedBuffer<MyConstData>,
    #[spirv(storage_buffer, descriptor_set = 0, binding = 1)]
    storage_buffer: &mut TypedBuffer<[u32]>,
) {...}

(comparable to buffer { uint data[]; } storage_buffer; in GLSL)

That is, plain Rust references are used to describe a single buffer, with the Rust type (MyConstData or [u32]) being for the buffer contents, used to interpret the raw bytes (provided to the host, e.g. Vulkan, API).

Because the "buffer handle" is made explicit, specifying multiple buffers is unambiguous:

#[spirv(fragment)]
fn main_fs(
    #[spirv(uniform,        descriptor_set = 0, binding = 0)]
    uniform_buffers: &RuntimeArray<TypedBuffer<MyConstData>>,
    #[spirv(storage_buffer, descriptor_set = 0, binding = 1)]
    storage_buffers: &mut RuntimeArray<TypedBuffer<[u32]>>,
) {...}

(comparable to buffer { uint data[]; } storage_buffers[]; in GLSL)

In other words, each TypedBuffer Rust value corresponds exactly to one buffer descriptor (e.g. VkBuffer).

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TODO(@eddyb): add changelog, tests, discuss UntypedBuffer (TypedBuffer<[u32]> wrapper), maybe deprecate ByteAddressibleBuffer (to rename it to ByteAddressibleView or something? unsure what's best here)