wpiutil: Change uv::AsyncFunction to use promise/future.

This allows the called function to pass along the promise to another
asynchronous callback.

To avoid memory allocations, add a home-rolled, simplified, non-allocating
version of std::promise and std::future as wpi::promise and wpi::future.

wpiutil/src/test/native/cpp/future_test.cpp

@@ -0,0 +1,84 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2018 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "wpi/future.h"  // NOLINT(build/include_order)
+
+#include "gtest/gtest.h"  // NOLINT(build/include_order)
+
+#include <thread>
+
+namespace wpi {
+
+TEST(Future, Then) {
+  promise<bool> inPromise;
+  future<int> outFuture =
+      inPromise.get_future().then([](bool v) { return v ? 5 : 6; });
+
+  inPromise.set_value(true);
+  ASSERT_EQ(outFuture.get(), 5);
+}
+
+TEST(Future, ThenSame) {
+  promise<bool> inPromise;
+  future<bool> outFuture =
+      inPromise.get_future().then([](bool v) { return !v; });
+
+  inPromise.set_value(true);
+  ASSERT_EQ(outFuture.get(), false);
+}
+
+TEST(Future, ThenFromVoid) {
+  promise<void> inPromise;
+  future<int> outFuture = inPromise.get_future().then([] { return 5; });
+
+  inPromise.set_value();
+  ASSERT_EQ(outFuture.get(), 5);
+}
+
+TEST(Future, ThenToVoid) {
+  promise<bool> inPromise;
+  future<void> outFuture = inPromise.get_future().then([](bool v) {});
+
+  inPromise.set_value(true);
+  ASSERT_TRUE(outFuture.is_ready());
+}
+
+TEST(Future, ThenVoidVoid) {
+  promise<void> inPromise;
+  future<void> outFuture = inPromise.get_future().then([] {});
+
+  inPromise.set_value();
+  ASSERT_TRUE(outFuture.is_ready());
+}
+
+TEST(Future, Implicit) {
+  promise<bool> inPromise;
+  future<int> outFuture = inPromise.get_future();
+
+  inPromise.set_value(true);
+  ASSERT_EQ(outFuture.get(), 1);
+}
+
+TEST(Future, MoveSame) {
+  promise<bool> inPromise;
+  future<bool> outFuture1 = inPromise.get_future();
+  future<bool> outFuture(std::move(outFuture1));
+
+  inPromise.set_value(true);
+  ASSERT_EQ(outFuture.get(), true);
+}
+
+TEST(Future, MoveVoid) {
+  promise<void> inPromise;
+  future<void> outFuture1 = inPromise.get_future();
+  future<void> outFuture(std::move(outFuture1));
+
+  inPromise.set_value();
+  ASSERT_TRUE(outFuture.is_ready());
+}
+
+}  // namespace wpi