[0010] Define casting rules for vk::BufferPointer<T,A>

[llvm-beanz]

Which proposal does this relate to?
0010-vk-buffer-ref.md

Describe the issue or outstanding question.
We need to define the casting rules and casting operators for vk::BufferPointer<T,A> objects. Completely off the top of my head, I think it would be beneficial to have a vk::pointer_cast<>() casting operator and enforce the legality of casts through the operator.

Open to other suggestions and explorations.

Additional context
See initial review discussion.

[s-perron]

If we use the idea for aliasing in #42 (comment). I would have no problem with adding a pointer cast.

There is not type based aliasing in spir-v, so this should not cause any subtle and dangerous interactions because of the different types.

If we go with @greg-lunarg's idea to add type based aliasing, then we will have to think carefully about this. My view would be to disallow it.

[devshgraphicsprogramming]

Can we re-use syntax we're all familiar with?

vk::BufferPointer<uint,4> pU;
auto pF = reinterpret_cast<vk::BufferPointer<float,4>>(pU);

Then pointer_cast<T,A> could just be a templated function for brevity.

Also if pointer_cast or reinterpret_cast allowed for taking uint64_t as a template parameter or function argument, then we could have support for "manufacturing pointers from uint64_t" but without the danger of implicitly making null pointers.

[BeastLe9enD]

Lets say I have a vk::BufferPointer that points to an uint array. Will it be possible to have a vk::BufferPointer<uint[]> and then use ptr.Get()[idx] to get the uint at index idx?
The reason I ask the question right here is because if that is not possible, I think it is very important to be able to cast from uint64_t to vk::BufferPointer<T, A>.

[greg-lunarg]

Based on requests from @natevm et al, I propose that we support a mechanism to cast from a uint64_t. The syntax would need to be able to specify the type that is being created. My preference would be the simple vk::BufferPointer<T,A>(u).

[s-perron]

Lets say I have a vk::BufferPointer that points to an uint array. Will it be possible to have a vk::BufferPointer<uint[]> and then use ptr.Get()[idx] to get the uint at index idx? The reason I ask the question right here is because if that is not possible, I think it is very important to be able to cast from uint64_t to vk::BufferPointer<T, A>.

You should be able to put the array in a struct, and then have a pointer to that struct.

struct T {
  uint[];
}

vk::BufferPointer<T, A>