Support asm!

[khyperia]

This blocks a surprising number of issues, so filing an issue to explicitly track and mention in other issues as "this is blocked on that".

Supporting asm! currently blocks:

• texture support: Texture support #204
• additional shader intrinsics: Support additional shader intrinsics #197
• barrier proposal (well, the implementation of the proposal, not the proposal itself): Add barrier proposal #216

There are alternatives to implement the above without asm!, but considering so many things would be resolved really easily if asm! was supported instead of the alternative of significant custom compiler support (I imagine quite a few future things will need it too), it'd be super great if we could implement asm!.

Supporting asm! itself is currently blocked by rust-lang/rust#78950, which once it's in (hopefully it gets in!), we'll have to wait for a nightly to drop with it.