[Fiber] Make isRunnable() perform a relaxed load and add documentation

It is safe to perform a relaxed load in isRunnable(), since the
information is potentially outdated immediately after it was looked
at.

Also, since Fiber::isRunnable() is a virtual method, keeping its
definition in the header has no real advantage, hence we move it out
of the header.

emper/Fiber.cpp

@@ -13,6 +13,8 @@ void Fiber::run() const {
 	function(arg);
 }
 
+auto Fiber::isRunnable() const -> bool { return runnable.load(std::memory_order_relaxed); }
+
 void Fiber::printTo(std::ostream &strm, bool withPtr) const {
 	strm << "Fiber [";
 	if (withPtr) {

emper/Fiber.hpp

@@ -128,7 +128,17 @@ class ALIGN_TO_CACHE_LINE Fiber : public AbstractFiber, public Logger<LogSubsyst
 
 	void printTo(std::ostream& strm, bool withPtr) const;
 
-	[[nodiscard]] auto isRunnable() const -> bool { return runnable; }
+	/**
+	 * @brief check if this Fiber is runnable.
+	 *
+	 * Checks if this Fiber can be run, i.e. if it still needs to be
+	 * run. Note that with multi Fibers, a positive result, i.e., if
+	 * this function returns 'true', may be outdated immediately due to
+	 * concurrency.
+	 *
+	 * @return 'true' if this fiber is runnable, 'false' otherwhise.
+	 */
+	[[nodiscard]] auto isRunnable() const -> bool;
 
 	[[nodiscard]] auto isMultiFiber() const -> bool { return isMulti; }