Build not dependent on code generation about rspirv

[kvark]

I like (2) - it's simple to put in: just check that git diff rspirv is empty after the code gen.

[antiagainst]

Agreed with @kvark. I think we can start with (2) first. Not opposing to (3) too. It's just right now the autogen stuff needs external SPIRV-Headers repo is used by both the spirv_headers create and rspirv crate. I'm not exactly sure how to structure it.

[Jasper-Bekkers]

Fwiw, as a user of this library I wouldn't be a huge fan of (3). This library already drags in a whole bunch of extra dependencies through num & num-derive that I'm not super fond of.

[kvark]

has the added benefit of getting rid of the weird triple build you have to do if you modify the autogen

Shouldn't it be double-build (instead of triple)?
Also, I think this is less of a problem now as #90 landed - autogen is fast enough that it's not a problem to run it twice. Only affects developers, which are... just 3.5 people at the moment :)