feat add futures

src/futures/future_test.cc

@@ -0,0 +1,10 @@
+#include <gtest/gtest.h>
+#include <sled/futures/future.h>
+#include <sled/futures/promise.h>
+
+TEST(Future, Test1)
+{
+    sled::Promise<int> promise;
+    sled::Future<int> future = promise.GetFuture();
+    promise.SetValue(42);
+}

src/system/pid.cc

@@ -0,0 +1,10 @@
+#include "sled/system/pid.h"
+
+namespace sled {
+pid_t
+GetCachedPID()
+{
+    static pid_t cached_pid = getpid();
+    return cached_pid;
+}
+}// namespace sled