Introduce AbstractFiber as superclass of all scheduled control flows

emper/AbstractFiber.cpp

emper/AbstractFiber.hpp

emper/Context.hpp

emper/ContextManager.cpp

emper/Dispatcher.hpp

emper/Fiber.cpp

emper/Fiber.hpp

emper/NextFiberResult.hpp

emper/Scheduler.hpp

emper/meson.build

emper/strategies/AbstractWorkStealingScheduler.cpp

emper/strategies/AbstractWorkStealingScheduler.hpp

emper/strategies/laws/LawsDispatcher.cpp

emper/strategies/ws/WsDispatcher.cpp