Add test program with fork helper

rt/test/fork.cpp

+#include <task.hpp>
+#include <fork.hpp>
+#include <join.hpp>
+#include <root.hpp>
+#include <cstdio>
+
+
+using namespace typon::rt;
+
+int fibo(int n) {
+  if (n < 2)
+    return n;
+  return fibo(n - 1) + fibo(n - 2);
+}
+
+template <typename T>
+Fork<T> fork(Task<T> task)
+{
+  co_return co_await std::move(task);
+}
+
+Task<void> hello(int i) {
+  int n = fibo(10);
+  (void) i;
+  (void) n;
+  printf("hello from %d on %u, fibo(40) = %d\n", i, Scheduler::thread_id, n);
+  co_return;
+}
+
+Join<void> parallel() {
+  for (int i = 0; i < 30; i++) {
+    co_await fork(hello(i));
+    printf("parallel resumed on %u\n", Scheduler::thread_id);
+  }
+  printf("parallel syncing on %u\n", Scheduler::thread_id);
+  co_await Sync();
+  printf("parallel resumed on %u\n", Scheduler::thread_id);
+  for (int i = 30; i < 60; i++) {
+    co_await fork(hello(i));
+    printf("parallel resumed on %u\n", Scheduler::thread_id);
+  }
+}
+
+Root root() {
+  co_await parallel();
+  printf("root resumed on %u\n", Scheduler::thread_id);
+}
+
+int main() {
+  root().call();
+  puts("done");
+  return 0;
+}