Languages that implement efficient coroutines (e.g. Scheme, Lua) usually manage the C stack themselves (like "Stackless" Python). First-class coroutines are very similar to first-class continuations, and many of the same fundamental issues manifest.
Lua has one of the best coroutine implementations I've seen. Scheme has continuations which are convertible to coroutines, while Lua has coroutines which are convertible to (one-shot) continuations, with community insight on their use.
Silent explained it all very succinctly. If you get confused about coroutines versus continuations versus generators, check out everybody's favorite coroutine manual: http://www.inf.puc-rio.br/~roberto/docs/MCC15-04.pdf
Lua has one of the best coroutine implementations I've seen. Scheme has continuations which are convertible to coroutines, while Lua has coroutines which are convertible to (one-shot) continuations, with community insight on their use.