[LAWS/Fiber] Introduce the concept of Multi-Fiber

LAWS, and similar scheduling strategies, only need to perform refcounting and atomic operations if the fiber was placed in multiple ready-lists in the first place.

emper/strategies/laws/LawsDispatcher.cpp

@@ -4,6 +4,7 @@
 
 #include "Common.hpp"	 // for DIE_MSG
 #include "Emper.hpp"
+#include "Fiber.hpp"
 #include "LawsStrategy.hpp"	 // for LawsStrategy, LawsStrategy::FiberSource
 #include "NextFiberResult.hpp"
 #include "Runtime.hpp"
@@ -12,8 +13,10 @@
 
 void LawsDispatcher::recycle(Fiber* fiber) {
 	// If the ref count has not reached zero yet, do not recycle the
-	// fiber.
-	if (decreaseRefCount(fiber)) return;
+	// fiber. But only if the fiber is a multi fiber, i.e. was placed
+	// in multiple queues in the first place, and hence needed to be
+	// refcounted.
+	if (fiber->isMultiFiber() && decreaseRefCount(fiber)) return;
 
 	Dispatcher::recycle(fiber);
 }