Regarding the idea of doing a typedef for the new coro type at the C level: looking further at the way the new type integrates with the eval loop, it's essential that they actually retain the exact same memory layout if we don't want to rewrite a whole lot of code.

I still like the idea of adding the typedef so we can at least make the distinction in cases where the C level handling *isn't* shared between the two types, it would just need a great big disclaimer saying that the memory layout of the coroutine struct can't be altered independently of the generator layout.