Builder next

[MatusT]

This PR corresponds to #141.

Current implementation:

• Each builder creates a trait ex.: ApplicationInfoBuilder -> pub trait ApplicationInfoBuilderNext
• Structs implementing this trait are then taken in the builder's next function
• Each struct then makes a bunch of impls for the traits based on the extends attribute provided in vk.xml

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This change is 

[Ralith]

Nice work! A couple thoughts:

• Don't we want the next values to have the same lifetime checks as other pointers?
• While it might be redundant to the unsafety of all(?) functions that process these structs, I think making the traits unsafe is appropriate to express the specific UB of passing in an incorrect one, bearing in mind that users may want to implement the structs manually to support extensions unknown to ash.

[Ralith]

Suggested change

	quote!{pub trait #name_builder_next {}}
	quote!{pub unsafe trait #name_builder_next {}}

[MatusT]

Up to this day (and I wrote about 20 000 lines of code in Rust for my bachelor thesis :D) I didn't know you could also mark a trait as unsafe. I leave this decision to @MaikKlein while I google away to learn about unsafe trait.

[MaikKlein]

I don't even think that a user would want to implement those traits manually, but if they do they better make sure it is valid, so yes it has to be unsafe.

Should this trait even be public?

Edit: Actually it probably doesn't need to be unsafe, because only the functions that use it need to be unsafe, but I think it is a good indicator to the user that they probably should be careful here.

[MatusT]

I agree with the indication of the fact that it is unsafe to implement it.

In theory it shouldn't be public but I can see that someone may be stuck for some reason on older Ash version in future but may want to add an extension from newer Vulkan.

[Ralith]

There's at least theoretically a population of users of unpublished extensions, too.

[MaikKlein]

This looks fine to me, is there anything that you want to add? I am just asking because there is still WIP in the title.

[MatusT]

@MaikKlein Nope, just forgot to remove WIP :). But let me just rebase and squash.

[MatusT]

Looks like one of nv extensions broke the next : /

[MaikKlein]

do a git submodule update

[MatusT]

git submodule update wasn't enough because before rebasing it was on branch, I hate git. Anyway, all is solved and squashed.