Adapt exception.cpp and forkjoin_fibo.cpp

rt/test/exceptions.cpp

@@ -25,7 +25,7 @@ Join<void> fail_join() {
 }
 
 Join<void> join(int max) {
-  std::vector<Async<int>> v;
+  std::vector<Forked<int>> v;
   for (int i = 0; i < max; i++) {
     v.push_back(co_await fork(fib()));
   }

rt/test/forkjoin_fibo.cpp

@@ -8,8 +8,8 @@ Join<int> fibo(int n) {
   if (n < 2) {
     co_return n;
   }
-  Async a = co_await fork(fibo(n - 1));
-  Async b = co_await fork(fibo(n - 2));
+  Forked a = co_await fork(fibo(n - 1));
+  Forked b = co_await fork(fibo(n - 2));
   co_await Sync();
   co_return a.get() + b.get();
 }