[WIP] cancellation support for async functions

[oberblastmeister]

If an async libuv action is running and libuv supports cancellation, the handle will be cancelled. This is supported for things like fs actions. If libuv does not support it, then it is still very useful because we perform a 'soft-cancel'. This means that whenever something yields, we never resume and just return, thus cancelling the coroutine. This can be used for things like timers and the coroutine abstractions in control.lua