Update readme and stubs

Update readme and stubs

Add ReflectionFiber::getFiber()

Add isStarted() method to Fiber and ReflectionFiber

Split ReflectionFiber into two classes

Update stubs

Drop Continuation object

Separate Fiber::run() into two methods

Allows for creating fibers without such deep nesting and adds ReflectionFiber::fromFiber().

Drop ReflectionFiber::getThis()

Add ReflectionFiber