neco VS libco

I don't know where you're getting that impression.

These are stackful, symmetric coroutines with a build-in scheduler. It implements suspend and resume. You can call that a green thread if you want, or a fiber or whatever. They're coroutines.

The API docs make all of this clearer than the README. https://github.com/tidwall/neco/blob/main/docs/API.md

Higan's coroutines library libco is low-overhead cooperative multi-tasking that avoids OS context switches. As it says in the readme:

Don't know about other systems, but bsnes/higan is probably the most widely known SNES emulator that operates at the cycle level. It uses libco to implement green threads. More details here, but afaik the basic idea is to emulate one subsystem for as long as possible until it touches another subsystem (like the CPU reading a register of the PPU), and at that point the other subsystem catches up.

Not sure exactly what you're looking for/what you mean by 'asynchronous/coroutine', but you probably want either libco or libuv (or both?)