Add future.cpp test program

rt/test/future.cpp

+#include <typon/typon.hpp>
+#include <cstdio>
+
+
+using namespace typon;
+
+int fibo(int n) {
+  if (n < 2)
+    return n;
+  return fibo(n - 1) + fibo(n - 2);
+}
+
+Task<void> consume(Future<int> & f) {
+  printf("[%u] waiting on future\n", Scheduler::thread_id);
+  int value = co_await f;
+  printf("[%u] future yielded %d\n", Scheduler::thread_id, value);
+}
+
+Task<void> produce(Future<int> & f, int n) {
+  int value = fibo(n);
+  printf("[%u] producing %d\n", Scheduler::thread_id, value);
+  f.put(value);
+  co_return;
+}
+
+Join<void> parallel() {
+  Future<int> f;
+  printf("[%u] fork(consume())\n", Scheduler::thread_id);
+  co_await fork(consume(f));
+  printf("[%u] fork(produce())\n", Scheduler::thread_id);
+  co_await fork(produce(f, 20));
+}
+
+Root root() {
+  co_await parallel();
+}
+
+int main() {
+  root().call();
+  puts("done");
+  return 0;
+}