[boost] [asio] simple coroutine lib.

I'm posting this under the asio header because it depends heavily on asio... I've uploaded in the vault a simple coroutine lib. Only works under unix though, but windows support could be added by some one with more knowledge than me of win32 fibers. In pratice the coroutine lib allow the creation of separate user contextes, and provides functions that return functors that when called reactivate the coroutine. This functors can be provided as callbacks to asio asynch functions. There is still a lot of work to do before the lib becomes actually useful, but it is quite good for just a weekend conding project. Plese, critique and comment :) --- Giovanni P. Deretta