feat add Promise

src/futures/promise_test.cc

@@ -0,0 +1,21 @@
+#include <gtest/gtest.h>
+#include <sled/futures/promise.h>
+
+TEST(Promise, Basic)
+{
+    auto p = sled::Promise<int>();
+    auto v = p.Then([](int v) {
+                  EXPECT_EQ(v, 1);
+                  return v + 10;
+              })
+                 .Tap([](int v) {
+                     EXPECT_EQ(v, 11);
+                     // no effect
+                     return v + 1;
+                 })
+                 .Then([](int v) {
+                     EXPECT_EQ(v, 11);
+                     return v + 10;
+                 });
+    p.Resolve(1);
+}