Move emper::sleep() implemention from header in compilation unit

emper/include/emper.hpp

 // SPDX-License-Identifier: LGPL-3.0-or-later
-// Copyright © 2020 Florian Schmaus
+// Copyright © 2020-2021 Florian Schmaus
 #pragma once
 
 #include <cassert>
@@ -10,7 +10,6 @@
 #include "Fiber.hpp"
 #include "Runtime.hpp"
 #include "SynchronizedFiber.hpp"
-#include "io/Future.hpp"
 
 void async(Fiber* fiber) {
 	assert(fiber != nullptr);
@@ -75,16 +74,6 @@ void yield() {
 	runtime->yield();
 }
 
-auto sleep(unsigned int seconds) -> bool {
-	if constexpr (!emper::IO) {
-		DIE_MSG("sleep requires emper::io");
-	}
-
-	emper::io::AlarmFuture::Timespec ts = {.tv_sec = seconds, .tv_nsec = 0};
-	emper::io::AlarmFuture alarm(ts);
-	int32_t res = alarm.submitAndWait();
-
-	return res == -ETIME;
-}
+auto sleep(unsigned int seconds) -> bool;
 
 }	 // namespace emper